My big advice--don't take too much. I have taken my boys to England, which is an 8 hour flight, and I always over pack. The worst thing is having to keep track of everything, and negotiate all the stuff. There is enough to deal with when you are traveling with kids.
Let your son choose a few things he wants, as long as they are small. If you have an MP3 player, put songs on it that the kids like. We just got one for my 7-year old for his birthday, and he chills out for quite awhile with it.
If you have a PSP or something that will play movies, we have done that. Compact DVD players are bulky and the battery life does not last very long. Plus, hauling DVDs is bulky.
Video Now systems are small, and you can get Spongebob and other popular movies. I would check Craigs List to see if there are any people getting rid of theirs.
You don't say how long the flight is. 2 hours is no big deal. (It feels like it when you are in it, but it really is tolerable) Eight hours is more difficult, especially with no sleep time, but just a blank paper and pen.
In short--be compact and conservative. Don't underestimate the fact that this experience is new and overwhelming, so that will buy you some serious time as well.
Good luck and have fun.