A good set of Exacto knives or equivalent would be a good choice. Many other uses for them besides building airplanes. He will also need some 5min epoxy, 30min epoxy, CA (basically super glue for planes) ,and a good quality wood glue like Tite Bond.
If your going to buy him a radio too, then you might as well just buy a flight kit similar to this:

https://www.towerhobbies.com/cgi-bin...?&I=LZ1427&P=T

This package gives him the radio system and all the other electronics he needs to control a Kadet or similar plane.

The O.S, .46 Ax would also be my recommendation for the Kadet. It's kind of like the "Toyota Camry" of engines. Not the most expensive, not the cheapest either, but probably the best value considering reliable quality and dependability along with good customer service, if it's needed..
There are some other things he will need after the plane is built like an engine starter, fuel transfer pump, fuel, and a few other tools. But I would wait about those until he is almost finished building the kit.

Out of HydroJunkie's list, two things I would go ahead and get up front is the Covering Iron and Heat Gun, All the other items should be supplied in the kit, so unless he looses or damages something, he won't need those clevises or fiberglass rods.