We took our kids on trips to England every year, so this old hat for me. :)
Mad Libs are fun, but may be difficult for him, since he probably doesn't know the difference between a noun, verb, adverb, etc. so you will have to assist alot with them, but he will love the silly stories that result!
We used to take playdoh with us. Just the little cans, in three or four colors. A plastic spork, knife, etc. work well as 'tools', and it cleans right off the tray table.
Bring a coupld of simple games to play. Uno, Pass the Pigs, Yatzee (you do the final math), etc. All three of those games fit into gallon ziplock bag together with a deck of cards for go fish, so they don't use up much space in your carry ons.
We would also bring a good book to read to them at this age. Something a little longer than his usual bedtime story. At his age he probably would like the Magic Tree House stories. They are too hard for him to read by himself, but they are very exciting stories when read aloud. There are tons of them, so you could pick two with topics he likes. Dinosaurs, dolphins, knights, egypt, etc. One for the flight out and one for the flight home. Just read a couple of chapters at a time in between other activities. Your daughter will like these stories too.
The last thing is snacks. You can't bring liquids or things like yogurt, etc., but you can bring peanut butter pretzels, cheese and crackers, granola bars, apples, etc. The flight attendants are happy to keep you in drinks. They know that a kid eating is not a kid whining!
Have fun on your trip!