The language itself is only one small part of the answer, and not the most important part. If he's having fun, he'll plod through a lot of crap to see the thing working. How many RPGs have mind-numbing resource grinding? Yet they're popular because, at the end of it all, you've achieved something you wanted.
So the question is: what should his first project be? A game that's easy to mod could be a good start. Last I checked, UnrealScript had great tooling and was used for lots of games. And a community that creates lots of cool stuff is Scratch.
In the end, your goal should be to give him "the bug." What's the bug? Here's how Richard Feynman described it, on some of the first computers ever during World War II:
Well, Mr. Frankel, who started this program, began to suffer from the computer disease that anybody who works with computers now knows about. It's a very serious disease and it interferes completely with the work. The trouble with computers is you play with them. They are so wonderful. You have these switches - if it's an even number you do this, if it's an odd number you do that - and pretty soon you can do more and more elaborate things if you are clever enough, on one machine.
But if you've ever worked with computers, you understand the disease - the delight in being able to see how much you can do. But he got the disease for the first time, the poor fellow who invented the thing.
And Fred Brooks, from The Mythical Man-Month:
The programmer, like the poet, works only slightly removed from pure thought-stuff. He builds his castles in the air, from air, creating by exertion of the imagination. Few media of creation are so flexible, so easy to polish and rework, so readily capable of realizing grand conceptual structures.