Hi there, and major respect for supporting your son's passion! I'm going to lend my vote to the drawing tablet. I have just a small one, and it's old, but it makes such a difference in how I interact with my drawing software. I think that would be pretty exciting for him to play with; it sounds like he's a master at learning acquisition, so just jump right in! You can find them at some computer and electronics stores if you don't want to wait for shipping.

Along with GIMP, another piece of software you might look at is InkScape. It works a bit differently but is really good for drawing lines and shapes and having a high degree of control. Both GIMP and InkScape are free, so you're only investing the time to learn how to start using them. My son has been using Adobe Illustrator (similar to InkScape) and I've been really pleased with how he's progressing.

There are lots of tutorials for GIMP and InkScape. I imagine your son is reading, and probably has been for a long while, so he will be able to take over his knowledge acquisition pretty easily and teach you!

Have fun!

Meshon