I am a mother of an addict. He stole from me, etc. I wish his dad would help me by having him pay me back. I am suing my son to get my money back, his dad said it will be a cold day in HE** before I get a dime from him. (My ex is a lawyer.)
If his parents keep bailing him out of debt, I say go and ask in a nice polite business matter show them proof of what you lent him.
The worse that can happen is they say no, and you are in the same boat as you are in now. Maybe they will be kind and help you even if they just give you part of what he owes you. I would also send a letter to the ex and list the amount he owes and ask for repayment, if he starts to repay you after his parents pay you, you could always send whatever he sends to you back to the parents. This way it shows them you aren't just after their money and since he is paying you are returning that amount to them. I don't think he will pay you back since addicts don't.
Good luck, let us know how you make out