will he watch a video? you can get portable dvd players... in the night garden was my dd1s first favorite show when she was just a bit older than he is, and baby einstein animals is my nephew's favorite.
buy some new toys for the plane and keep bringing a new one out when he gets bored with the first one.
you can do a lot of walking up and down the aisles and if it is a big plane you can hang out in the back, there's usually a bigger area.
if it's a long plane ride you may want to book an extra seat even though you have to pay for it when he's technically free under 2. or ask when you get on to be put next to an empty seat and you may get lucky. i found it pretty claustrophobic having dd1 sit on my lap for a trip from BC to NS when dd1 was 11 mo. luckily there were a couple legs of the trip where it was not sold out and we had an empty seat next to us. and one of the legs we had the bulk head seat which was really nice - try to book that one if you can, there's way more leg room and he can crawl around on the floor at your feet.
