I'll give you my OPINION, but don't take it as ADVICE -- because I have a very hard time letting things like this go but, the older I get, the less I think that's a good characteristic.
Write him a demand letter. Either send it certified mail, or make him sign a copy when you give it to him. Give him a deadline, or a payment schedule you'll accept. Don't get emotional in the letter, just be matter-of-fact - you could even sound a little "gentle," which couldn't hurt you in the next step. If he doesn't pay, take him to small claims court. It'll cost you, and it's a stressful process, but it's holding him to his obligation.
I have been lucky to have never been in a FINANCIAL conflict with an addict, so I'm probably not the best person to offer an opinion on this...
KayM