I'd favor the LEGO one, because that also builds to skills for robotics competitions and needs specialized hardware to do and is not easy to troubleshoot oneself; does he do LEGOs at home?. With Scratch, if it interests him, it is free at home and fairly intuitive to use: if he likes it, odds are he is up and running and doing stuff in an hour on his own, if he doens't odds are the class isn't that much fun. Minecraft is pretty clunky to do progammatic work in or game design, I'd speculate it is more soft skills in level design than re-useable programming type skills; Minecraft also has such a rich Youtube presence and community, a class seems a bit counter-intuitive.
