Hi there. I've lost count of the amount of times I've traveled with babies and young children. So here are the fruits of my experience.
Make sure to nurse him on take off and landing. The swallowing motion will equalize the pressure in his ears. It's actually easier to travel with a baby if you're nursing. That way you don't have to carry around anything like water bottles and formula, clean and dirty bottles etc... It's all right there, just the right temp, already perfectly mixed and "on tap" for baby.
In September your son will be more mobile and better able to feed himself finger foods, so take along a lot of wholesome snacks, in a small cloth cooler, that you can keep him busy with. My favorites include dry cheerios, fruit bars, dried fruit (you can soak it for a minute in one of those little plastic cups of water they give you on a plane to make it easier for him to chew), pb and j sandwiches (if he's eating peanut butter by then), string cheese etc.. Remember the string cheese because kids get cranky when they're low on protein.
Ditch the purse. Put your important items (wallet, phone, tickets etc.) in a backpack. Fill it the rest of the way up with diaper bag stuff. Pack your purse in your checked baggage and leave your diaper bag at home. Use the backpack as both. Pack extra diapers in your checked baggage or just buy a new pack when you get there. You'll probably also have room in the backpack for books and toys. I recommend taking one old favorite of each and all the rest should be new ones that he's never seen before. Don't forget his comfort object if he has one and a light blanket.
If you have a Boppy, take it with you! This will make you and baby sooooo much more comfortable when nursing on the plane and he can take naps in your arms without putting strain on you if you have this to keep on your lap under your arms.
Don't stress about anyone seeing you nurse him. You're never going to see those people again, so who cares if they watch you nurse. Don't make yourself and your son more uncomfortable by trying to find a private place to nurse. Relax and do what you need to do. Use his light blanket to cover yourself if you feel the need.
Make sure you're both wearing comfortable clothes. I personally travel in warm-ups, t-shirts and flip-flops (a must to make security easier) with a simple pony-tail that I can pull out so it's not in my way when I need to take a nap and can throw back in easily when I'm ready. Make sure that baby is in equally comfy clothes. Take off his shoes before you even go into the security check-point and get all the way to the gate before you stress about putting them back on. Baby's clothes should be soft and comfy jersey knits (t-shirt/sweatshirt material). Nothing binding and everything easy to move around in.
Don't forget that airline travel is dehydrating, so take an empty water bottle that you can fill up from a water fountain once you get through security.
Take a notebook of regular paper and a pen. This can entertain a little one for hours, especially if you are willing to draw silly pictures for him also.
If you have one, use a baby carrier for the airport, instead of a stroller. I always found that my trip was easier if I used my hiking baby carrier backpack and slung my diaper bag/purse backpack off my front. This way baby was on my back and all of the things I needed access to were in front of me and within easy reach. It's a bit of a pain getting through security, but no more so than a stroller and you can gate-check it just like a stroller.
When you go to change him in the airplane bathroom, there is a changing table that comes down from the wall, but it's really hard and flat, so take the blanket (folded up) with you in there for a little bit of padding to lay down under your waterproof changing pad. Don't take the whole backpack in there with you either. It's difficult to maneuver in the aisle with a lot of stuff, so just take baby, one diaper, blanket, changing pad and the packet of wipes in there. Don't forget plastic shopping bags to wrap up stinky diapers in before throwing them in the garbage.
Let the baby's schedule go out the window. If he wants to sleep more on the plane then let him. The more time he spends sleeping on the plane the less time he'll spend awake and needing attention.
If your son is walking by that time, then don't be afraid to stand up with him and let him walk up and down the aisle (hold his hands though for balance, you don't want him to hit his head on the armrests). At the back of the plane is the area that the attendants move around in when they're getting service carts ready. Most attendants are very tolerant of moms/dads taking small children back there to stretch their legs a tiny bit as long as they're not using the area at the time. If he gets restless, take him back there and let him walk around.
Well, that's all I can think of right now... I'm sure you'll get lots of other good suggestions and tidbits of advice. Here's to hoping your flight isn't full and you guys get an empty seat next to you! :)
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Edit: Just wanted to add something about the bulkhead seats. This is no longer an option unless you want to pay more. Most airlines now have three zones in the airplane. Economy, Economy Plus and First Class. Economy Plus includes the bulkhead seats and it costs more. These seat are placed a little further apart for extra leg room. This zone usually includes the first several rows of the coach cabin. JSYK.