the same as you do at home, minus the running around. Take something he knows well - favourite blanket, say - and a few small favourite toys - books, crayons or whatever. He won't know he's in a plane. Have something for him to suck on, a bottle of milk perhaps, when taking off and landing so the change in air pressure doesn't hurt his ears.
Drugs - it's the only answer! Try and keep baby awake until flight time so they'll sleep at least part of it. Medised is good for making them sleep (or just Calpol). We flew to Florida with my 15 month son, who wasn't quite walking, and I never sat down for 10 hours. The crew were very good and didn't seem to mind.