There are tons of components around that can be interfaced to an Arduino, but my personal opinion is that perhaps is better to postpone the purchase to the time when your son will clarify to himself what he likes to do most with his new toy :-)
Another idea: when I started with Arduino, I found myself desperately in need of things like pliers, small scissors for wires, tweezers and - most of all - a cheap but reliable multimeter with a continuity test. If he doesn't have things like that, they can all be nice ideas for a complementary birthday present.
Just throwing ideas around...