Airplane Toys

Silly putty
Sticker book
Hand-held fidgets (do an internet search--there are many varieties available)

My kids loved sticker books at that age. Try to find the reuseable sticker books. They aren't really reausable, but you can unstick and move the stickers around a few times.

My kids also had fun with pipecleaners, bending them into different shapes. Just be careful because in some brands the wire end sticks out and can be sharp.

But, like others have said, an iPad or similar device is a great thing to have as well. We just flew and my 4 year old kept busy with that for most of the flight--drawing app, coloring app, movies, read-to-me books, educational games, etc.

DVD player w/a few of his fave cartoon movie (you can turn it on low vol)
Dry erase board & dry erase markers
Some blocks. Bring a small shallow wicker basket or tray for him to use
as lap tray to put blocks together. The basket lip will help keep them
from falling over
A few new small books
The key here is NEW toys, gadgets he's never seen before.

my son was easily entertained by a whole pad of post it notes, pipe cleaners, small paper plates and cups and a roll of masking tape, a plain roll of tape, a cheap deck of cards he can write on or color (playing or picture cards), strips of Velcro taped to strips of felt, mini Rubik's cube, a small toaster oven size pan and any kind of magnets, a sandwhich bag of colored paper clips, and a bag of small containers of playdoh with slim popsicle sticks to stick in the playdough.

I gave him each activity one at a time and they worked wonders at that age on the plane, in the airport and long road trips. I hated packing all the heavy and bulky plastic and wooden toys on long trips, and a bag of these disposable items worked great, because at the end of the trip, all I had to do was toss them in the trash.

Etch and sketch, if he still enjoys it, would be a great one. I also would recommend some drawing materials (washable markers/crayons, a couple pads of paper). A small white board with a couple dry-erase markers might be great too--hang-man or other writing/drawing games. Does he enjoy cards? A deck of cards for go-fish might be good. Also, depending on how long the trip will be (e.g, overseas), if you will have access to a tv-video screen, you might consider relaxing screen time restrictions for the flight. We have taken our kids to Tanzania numerous times, and frankly, the individual screens were a life-saver. Have fun!

Definitely a tablet of some type. Load it up with a couple of movies, games, kid's books, kid's music, and plug the little bugger in for the ride. An airplane ride is the exception to every "don't give your toddler video games" rule.

We travel a lot by plane with three kids. Depending on the trip (length of flight, type of/length of trip) sometimes my two bigger kids pack their own carry on which will contain books, colouring/puzzle book, DS, iTouch, Legos or Littlest Pet Shop, stuffed animal, blanket. I will bring books, tablet, colouring book, a few cars for the little guy. This last flight was only 2.5 hrs and the trip was filled with swimming and playing with friends, all I packed in my carry on was the 3 iTouch. I packed a book for reading before bed in the suitcase.

My almost 4 yr old loves movies now. We have a LeapPad for her that she will watch and play games on for an hour at least.

She also loves to draw and color.

So I would take a look at what he will really focus on, and start there. It doesn't have to be new things at that age. Just things they like.
Is he into legos at all? Magnets?

We got a sticker book (the kind with reusable stickers) and it was pretty popular. It was from Melissa and Doug and had several different scenes, each with tons of different reusable stickers to use and rearrange. That plus the iPad for a movie were the favorites

We made puppets with the air sick bags. We had a magnetized doll set up ( like paper dolls but on a magnet board).

Our daughter is 19 and we started flying with her at around 4 months. We fly quite often. I kept lollipops around for take off and landing when she got older. The ear plugs ( ear-planes) seemed to work well for us.

Her most favorite was 1st class to Hawaii ( before 9-11) with attendants dancing and make your own ice cream sundaes! After that flight, she always requested the ice cream section!!!

Have fun!

Updated

We made puppets with the air sick bags. We had a magnetized doll set up ( like paper dolls but on a magnet board).

Our daughter is 19 and we started flying with her at around 4 months. We fly quite often. I kept lollipops around for take off and landing when she got older. The ear plugs ( ear-planes) seemed to work well for us.

Her most favorite was 1st class to Hawaii ( before 9-11) with attendants dancing and make your own ice cream sundaes! After that flight, she always requested the ice cream section!!!

Have fun!

If all else fails, the plastic cups that the flight attendant passes out w/ drinks makes an excellent toy. If it rolls under the seat (and it will) - not a problem.

At that age, we brought a lot of books - we read to him or he looked at the pictures. Thankfully he slept a lot. And yes, the plastic up is a life saver.

Portable DVD player OR download kid shows onto your tablet if you have one
Leapster (or whatever the newest version is of their handheld game)
Headphones - because ear buds definitely don't work for kids, and everyone around you doesn't want to listen to Doc McStuffins :)

iPads or DVD player is a great thing. Episodes of his favorite tv shows might be better than full-length movies at his age. For my son, hot wheels would be good once we got to our destination but too exciting for the plane. Little plastic dinosaurs or farm animals are good since they're easily replaced. Books are good, too. Don't forget suckers to help with the pressure changes. Have a good trip!

For me, the thing would be that I would NOT want to pack... a whole other bag to bring on the plane, just for toys/entertainment nor it being bulky or heavy to carry all that and I would not want to bring things on a plane for kiddo that has a lot of pieces to it that can and will get lost or what have you.
So, an iPad. Does it all. And you adults can use it too.
If you already have one that is.
Or use your iPhone. If you have one or Android. He can play on that. Too. With apps that you ahead of time, load onto your gadget.

if you have access to it a portable dvd player is great. or a laptop you can play dvd's on. you don't say how long your flight is (hoping not 3 weeks lol) but when my boys were that age we took a portable dvd they watched a movie and then color books with colored pencils (they don't melt in the rental car in the sun later) and sticker books. they loved sticker books. they also had hand held games (gameboys they were called then but I think they call them nintendo ds or dsi's now) books on tape / cd are also good. let him have his own backpack with stuff in it that he can carry. and let him help choose the toys. nothing big. nothing messy. nothing loud. make sure if you bring the dvd thing that you bring ear phones. my grandkids had a dvd player last year and the man behind them said it's too loud even though it was on the lowest setting. so be mindful of that.

My mother used to pack us bags of activities every year for our cross country trip. Her trick, as she told me, was to pack things we liked doing every day along with some cheap new toys (I think it was always just dollar store stuff) and snacks that were buried under all the other things. But she made sure she had at least one thing for every half hour we would be on the plane. That way we would discover our way through the bag on the flight and always have something new and fun to do for half an hour before we got bored and moved on.

If he likes to color the color magic markers are nice or the triangle shaped crayons so they don't roll away. I usually take several books, ones with flaps are always fun. Wrapping a few small new or toys that have not been used in awhile. Dvd player or tablet with TV shows if he likes any. Stickers are fun with paper. A few duplos. My year old loves to take a toy plane when we go, hot wheel sized. Lots of snacks, some sort of soppy cup or one with a lid if you. Most airlines don't have lidd, a few do. I usually try and take something for every 20 - 30 mind and rotate as needed. Toy camera or a real one is also a lot of fun.

DS, iPod, iPad and books.