Don't pay him in advance for work, aside from that what he does in his personal time isn't really anyone business but his own. I'd have a backup plan though. Cause when he dies, or goes to jail, you'll need another coder, and if he's been hitting it for 5 yrs one of the two is gonna happen soon. You don't occasionally smoke Meth. So his habit is only gonna grow, so figure 5 yrs he's probably got quite the habit
Good luck