We've flown 5 times with my son since he was born, and he has always flown as a lap baby. True, I have always flown with him and my DH, so I had help, but it is not as horrible as you might think.
Of course, my child is most certainly different than yours, but do not fear! Just be prepared with reasonable expectations.
It sounds like your guy needs room to move. There is often an empty seat next to you (for instance, if you are on Southwest, no one sits next to you bc you have a toddler!) so you might have a bit of legroom. Or, to make sure you have some room (and if you are not flying Southwest), you can call the airline to ask for a bulkhead seat, which will give you more floor room.
I have a friend who regularly travels (Europe to the US and back) with his 2 under 2 (!) and he says the best thing to do is set up a play area on the floor. That way the child/ren have a bit of room to move around and play. Please note: there are no diaper changing stations on airplanes, so you will have to do that on the floor or on the seat. Also, try to figure out how you will take care of your own needs on the flight (going to the bathroom). You will have to take your little one with you! My friend says he just never gets to go when he's on a long flight with his daughters.
But it could very well all be fascinating to your son, who will suddenly be quiet and sitting still for longer than you ever would have thought him capable.
Bring toys that he has never seen before. Try letting him watch the tv. Bring favorite books/toys. Give yourself a break by being prepared for the best and the worst.
You can do it!