I truly believe that the drug use is a symptom of an underlying issue that somtimes even the addict is not aware of.
We all have things in our lives that are difficult to cope with...that cause us pain. For some, it's easier to just escape.
I have two children (grown) that were both raised in the same environment, have the same "bumps in the road" in their youth (like the divorce). One is an addict, the other isn't.
Rehab is great for teaching an addict coping skills (i.e. situations that will cause them to use = "triggers", how their mindset works, etc.). Then it's up to them from there. Like the others said, I think only time will tell.
If it were up to me (in a perfect world), all addicts would receive intense personal counseling. If we had the time and resources, we would dig and dig and dig until we found that issue (or issues) that made them feel the need to use in the first place.
I think the best thing you can do, since there isn't a hard and fast answer to your question, is to have that "actions speak louder than words" attitude. And please remember you can't control this thing...it's entirely up to him. You can be supportive, but don't run yourself crazy trying to figure out whether he's using again or what you can do to ensure he doesn't. It will make your life the most chaotic H**l you've ever been through.
Good luck and stay in touch. We're here for you.