Travel is so fun and exciting!
I've taken my daughter on many flights just the two of us with very little problem.
Like Christy I bought some special toys and books just for the airplane. We colored, drew pictures, played with dolls, looked at I Spy books, did sticker/activity books, made figures with pipe-cleaners, and read new stories. Those magnetic drawing boards and aqua-doodles are great too. You could even bring a little leggo set. I let my daughter pack her own little carry on bag with her special toys, favorite snuggle toy, and a couple of snacks in ziplock bags that she could open herself. Of course we took lots of little walks in the airplane to the potty too.
A DVD player might be a great thing to have for the very long flight. My daughter loved the Wiggles when she was a toddler. They have a lot of DVDs and CDs out.
I think you can buy milk once you're through security and bring that with you on the plane. I believe that 3-oz jars of baby food are allowed, but I would call the airport just to be sure. Maybe some new/special snacks would be a hit on the airplane. How about rice cakes, a new kind of cereal, freeze dried fruit, or different colored potato (or other veggie) chips?
I think you can still use your big stroller and take it through security right up to the gate. You can leave it just before you get on the plane and pick it up right after you get off. It might even be easier to have that big one so you can load it up with carry on bags, a drink, diaper bag, etc. and still have hands free for your son. If you use a smaller stroller, you have to carry more. I highly recommend you use a backpack as a carry on.
I made sure we ran around and jumped up and down, etc. as much as possible before we got on the airplane. Even though we may have looked silly, I think it paid off. Also, try to schedule flights close to his usual nap times.
I actually found not bringing the car seat on the plane was easier once my daughter was older. They let us check it without an extra fee, and then I didn't have to lug it around the airport until we boarded or during layovers. We purchased a carrying case for the car seat, which was very helpful and kept it clean. Then we could put the arm up between our seats on the plane and she could lie down on my lap and stretch out.
I remember it was important for her to be awake for take off and landing so her ears could adjust. Instead of gum I allowed her to have a couple of TicTacs or a lollipop as a treat so she would be swallowing to help her ears. (When she was an infant, I just nursed her, which was great! If I was next to someone, I always asked first to make sure no one was offended.)
The most important part for me was talking to my daughter ahead of time so she knew what to expect. She knew the basics of what would happen when we had to go through security (like taking off her shoes and putting her bag in the bin), the importance of staying close to me in the airport, having to sit for a long time, the possibility of loud noises, and most of all the excitement of travel.
I hope you have wonderful and safe trips!