The Korg ER-1 is a very intuitive piece of kit, but as you say, it's quite limited soundwise.
What about some loop-based software that lets him quickly put a song together, but can also allow him to make his own patterns? My suggestion would be
Cakewalk Kinetic 2. It's very intuitive, easy to use, and comes with a decent selection of softsynths, including the Roland Groovesynth, which is basically a rompler with many sounds to get him started. Once he gets the hang of it, he can start messing with PSYN-II (which is based on z3ta+), and the DropZone sampler. Best of all, it's relatively cheap and works pretty well on any PC.