The last time I flew with my first son he was just over two years. We brought his Britax Roundabout Car seat and strapped him in. Even at 3 years I would probably do the same because it made him less mobile, I felt he was safer, and I think it kept him in his seat as apposed to just the airplane belt. He did great this way!!
We brought a portable DVD player with earphones so that he could watch his shows. This worked great, but I do suggest you try the earphones before the trip so that he can get used to them!!
Oh yeah, I also suggest that you bring plenty of his favorite snacks. This was also a lifesaver, especially during those times you couldn't have the DVD playing!!
Good luck!!