I thought he wasn't able to do the rehab program due to insurance reasons? Did he just complete the detox portion? If so, he will probably still need to attend some sort of program, NA, etc. to help him learn how to make different choices and deal with those triggers in ways other than using again.
I think it's great to be positive, and it can't hurt his attitude, but if I were you I wouldn't necessarily bet on him staying clean forever from here on out. Hope for the best, plan for the worst? I'm still trying to get my head around that and actually put it into place. It's difficult, because you just want to try to think positively all the time. The chances of him relapsing are so high (unfortunately) that I don't think it will do you any favors to be OVERLY positive.
I also think it ca be trying for more than just the first few months. Through the journey I'm on, learning more about my bf's addiction, I've talked to many people who were clean or whose loved ones were clean for YEARS and began using again. It's a rough road to stay sober.