I would never lend someone money outside of family or my couple very best friends. But you've probably already figured that life advice out for yourself by now.

What's his mom like? Can you tell her? Sounds like a childish move but I bet that works better than any other legal routes you can take over it. I've known a few people (when I was in my early twenties when quite a few people still lived at home) that told the delinquent's parents. Once the parents got involved it was game over. But this depends on the parents giving a fuck of course.