Don't let him out of his seat to run around as he may want to keep doing it.
Don't give him too many toys as it will make him to excited, you want to get him in a slightly bored, zoned out mood. Just act like you find it a bit boring yourself, keep everything low key. The hum of the plane should lull him to sleep if you are lucky.
Don't take round toys that can roll away if dropped.
Take all your own food and drink for your DC then you wont get frustrated if the air stewards are not attntitive enough.
Take ziplock bags for dirty cloths
I used to fly on my own with three DC's under 4 on long haul flights (11hours) and I sometimes got them checked out by their doctor in case they had unsymptomatic ear infections. ( One of my DC's used to get ear infections from time to time)
On my doctors suggestion I sometimes used to give my DC's travel sickness medicine. It stopped them getting airsickness and knocked them out and helped them fall asleep. I only did this on longer flights.
I can't think of anything else 
Good luck with your trip.