[technomancy]

You should take a look at Minetest: http://minetest.net

It's a bit less polished, but far, far better when it comes to hacking. I found it significantly easier to create a mod for Minetest than to use a mod in Minecraft. Once my kids grasped the idea that they could add whatever items and behavior they could dream up, they really took a shine to it. The Lua coding used is very approachable.

[wjh_]

That does actually seem like it might be better, I admit Forge and Java would probably seem quite daunting to someone new. I'll see what he thinks!

The only issue though, is that I think a lot of his motivation stems from potentially sharing what he has made - pretty much everyone his age has a copy of Minecraft.

[technomancy]

There is a bit of a network problem with it; sure.

One advantage that may help is not needing a license; if you have siblings that have to share a Minecraft account they can play Minetest on as many machines as they have without bugging their parents to pay for a new account. Plus it can run on much older machines.

But yeah, it can be a tough sell just because people don't want to bother.