First off, congratulations on actually trying to help him (instead of dissuade this idea).
As your son is 12yo, and he'll probably start working (if everything goes well) around 18, he'll have a lot of ground ahead of him.
I say this because some of the technologies we use today to make games actually came to life in the last 6 years, so it is possible that some of them will fade as well, giving room for something even better.
This takes me to my point: don't focus on one specific technology, but on the general knowledge for now.
Some programming basics with Python or other higher level language would be a great starting point. You could take a look on pygame, or even Flash, to create this basics on what's a game, how it works by a bird's eye and then move up to something more complex.
This way he can then decide if he actually want to be a programmer or a game designer or maybe something else; as he probably doesn't understand the difference yet, as I didn't when I started as a kid.
I don't have anything to add, but never do Flash it's a technology that should die, it's a resource hog and has horrible performance even on high-end machines. Hopefully it will be replaced by HTML5.