Yeah, since I'm pretty sure no one can die here.

[... That's a good question.]

For starters, make him clean up the mess he made. Have him repair stuff by hand - not with his Pokémon. Teach him a skill that he can use and get up close and personal with the people he hurt. It might not work, since I don't think anyone who blows up buildings is really totally sane, but going face to face with the people he's hurt might get the guilt to start kicking in.