Dorton, I think you have been thinking about what may be the next best choice for you. I really like the idea that you did not give a reply right away and that you and your husband are going to discuss it. I also like your thought about asking him what he is willing to do...This isn't your "problem" It is his and so whatever happens, he has the power to make the change.
I understand how difficutl this can be. My daughter lived with us and the first time she went to rehab she wanted to return here. We established boundaries but those boundaries were challenged on and off as she struggled with recovery. Early recovery is tough...it isn't a straight line most times, and being a 19 year old even without addiction can be a challenge. Although every time she slipped she got back up and kept trying (and that was one of our boundaries...she could not stay with us if she wasn't working a program) I was still riding the roller coaster. I was early in my own recovery...just starting Naranon, a few months posting here. Like my daughter, I was green and having my own codie relapses.
Her second rehab we said no coming home - halfway house after rehab. She understood and actually believed it was better for her too. Unfortunately there was a waiting period and she came home to wait for 2 weeks. I wasn't comfortable with the idea, but her counselor thought it best. It really wasn't...
For me, it was easier to say no coming home when she was out of the house than it was to say "you have to leave."
I wish you and your son all the best. I hope his email is the first step to wanting to do whatever it takes to live a clean and sober life. Hugs