Was he a self referral to rehab? If so, that's great!...meaning that he recognized he needs help and wants to become sober.
The only thing I can offer is to be friendly, supportive, but most importantly, don't take his continued sobriety onto yourself as your responsibility. If he isn't able to remain sober, that's his cross to bear...you must do your best to not blame yourself if he has a relapse. Self, always...them, whatever you can spare after self.
