Hey, I'm not sure if it's been suggested yet, but if your son's super interested in game design, you should check out
the RTB (Return to Blockland) wiki. It's got information about playing the game, making 3D models for the game, and scripting for the game.
You should also check out/get your son to check the
RTB listings forum. It's got a list of tons of Blockland add-ons, all quality checked and checked for non-bugginess. It's complete with ratings etc. Just use common sense (don't enable two zombie mods at once etc).