Does he have a DS or other handheld device? Books that he would enjoy reading or puzzle books/activiity books? favorite movie on the ipad or other device? Any new toy he would enjoy playing with? Favorite snacks or gum that's a treat, that he can only eat at certain times during flight?
What small things would he enjoy having? I guess it depends on how long your flight is. My kids, a few years older than yours, have always found something to do. I even played hang man and tic tac toe with my DD when she was younger.
You can also bring a map and guidebook or other and have him review where you are going and what you are doing and ask what he wants and talk about your destination with him.
I hope your DS has fun flying. My kids have flown so much, they just chill on the plane most of the time now.