I have flown many times with my little one - 'still' is and always has been an alien concept for her! Flights were definitely a challenge as I was usually on my own so I can sympathise. Here are a few things that helped me:
Beg and plead for an empty seat next to you - this is a huge help. He will still have to sit on your lap for take off and landing but inbetween he will have much more space - if there is one on the plane then they usually oblige. If check-in won't give you one then ask again at the gate and then loudly when you get on the plane (so that other passengers hear you and hopefully someone will offer to move).
Wrap small toys (don't have to be new) in several layers of wrapping paper/ newspaper - trying to open them keeps them occupied for a bit (every second counts!).
Stickers - lots of them. Try and get ones that are the re-useable type so that he can stick them on the back of seats / windows etc.
Take lots of snacks and drinks for them to munch through. You could wrap them up or put them in little bags for them to fumble with.
Playdough - you can actually buy mini pots of it, perfect for travelling. Together with a new thing to press it into or squirt it out of.
Exploring the plane (if they will let you) - my daughter loved walking up and down the aisle talking to all the other passengers (who were only too happy to be distracted from their own boredom).
Can you borrow, or do you have, a protable DVD player? (I never had one but wish I had..).
Don't be tempted to give him any kind of medised or medicine that makes him drowsy unless you are sure it will make him sleep - I tried this with my daughter once and it just made her very tired and very very pissed off - not a good combination. It was the worse flight we ever had.
I think if you are very prepared and kind of accept the fact that it is going to be difficult then it is usually easier than you think. I also found that a glass of wine (for me!) and repeating 'this too will pass' over and over again was very helpful too! good luck.