Suz, maybe you are feeling these emotions because it didn't happen when you were married, it happened after the divorce? It's natural to feel emotional when they finally reach out for recovery. Rehab is not a cure, but I hope he embraces all they teach him and finds a better path for his future.
For you, your part is whatever you want it to be. If you want to detach from the whole thing, that's okay. If you want to support from the sidelines, that's okay too. Perhaps let him get some solid recovery under his belt before you commit to anything more than a supportive friend.
It's entirely up to you of course, but that's my point. You get to decide how you live your life. Sounds like you've done well so far.
Hugs