I have to agree wtih these moms about a backpack with stuff that's new to him (small books, toys that are inexpensive) and a portable DVD player will be worth it's weight in gold. We flew when my now-teen was 3 - she was very active, the flight was in the evening and was then delayed.... She didn't fall asleep until the very end of the flight - but she kept busy with other stuff (they didn't have portable DVD players then!). When she got bored of one thing I'd pull a new item out of her backpack. They were all inexpensive - but stuff that was completely new to her.
I wouldn't suggest that you allow him to get too tired in hopes that he'll sleep on the plane. he might jsut get over-tired and cranky. You know your kid best.
Set expectations for him. Tell him that flying is a grown up thing and tell him about the grown ups that will be sitting around you (when you get on the plane remind him). Explain that's very few kids and babies get to fly becuase many of them can't stay quiet and busy on their own. Tell him that you know he can be quiet and stay busy - and that you're going to have things to help him stay busy. Then praise him as he does what he needs to do "you are doing great, I bet the grown ups around us are glad that you're not babyish and that you're behaving so well!" If he slips and gets a little loud remind him of the people sitting around you - how they want to have quiet kids around them. Engage in games with him ("I spy with my little eye something red..." or make up word games "if you could do anything in the world right now what would you do?" tell him what you'd want to do (i.e. "make a giant chocolate cake and jump into it like it was a swimming pool").
As another mom worte - get to the airport with enough time to let him stretch his legs and run. Bring some cash so that when you get to the other side of TSA you can buy a drink. They will let you bring gum or candy in your carry-on - and even if you don't otherwise allow candy/gum this would be a time where you can relax the rules. Only when you feel the wheels actually lift off the ground do you give your son his special snack. Explain ahead of time that when you go up high pretty quickly the air between behind his ear needs help moving around so we can help that ir move around by swallowing - so you'll get him a drink or a special snack. Kids like to understand waht's going on - and explaining it ahead of time will help him not to be freaked out.
As for motion sickness - I used to get this all the time. People would tell me to read or keep busy with stuff. What I learned is that the reading the reading and keeping busy with my hands often made the motion sickness worse. Ends up that when in I'm in motion and trying to read my eyes have to keep re-focusing and the message being sent to my brain doesn't match the motion that my body and inner ears are feeling - so I get that feeling of nausea. For me, I do better when focusing on things that are moving at the same speed I am - so on a plane that would be the people around me, the flight attendents, the signs on the wall, etc. Other people do better when they focus on the horizon - so looking out the window at the skyline, the clouds, etc. Try to figure out what works best for you.
You and your son will do fine! And so will your mom. My husband had spinal surgery last Summer and the recovery was remarkably quick - so so so much easier than my mom's had been years ago at the same age - like 20X easier!!! They use such minimally invasive procedures these days.
Good luck Mama - you'll get to the other side of this and be amazed at how well it all went!