maybe iDrum? (
iZotope, Inc - Audio Processing Technology and Plug-Ins for Pro Tools, VST, MAS, Audio Unit, and DirectX) it's really quite simple and might be good for him.
either that or FL studio. can be as simple or complex as you want. FL studio kind of looks like a video game too, so that might be fun for him.
you can drag sounds to the step sequencer and click to make noises...or use the piano roll and have a keyboard to lay out notes.
includes all manners of sounds to get him started.