I think that this is the best choice. A lot of people are recommending "real" (albeit simpler) game making tools, but I think that those would be a mistake. At 12 his biggest obstacle is going to be becoming bored, not the skill ceiling of the tool. Scratch provides that instant visual output, skips the frustration of learning syntax, and has a large online library of other's creations for inspiration and learning.
Side though: scratch style programming inside minecraft has the potential to be awesome educationally. Somebody steal this idea (unless it already exists).
Side though: scratch style programming inside minecraft has the potential to be awesome educationally. Somebody steal this idea (unless it already exists).