"I understand boundaries and what the future might be."
What boundaries did you and your husband create before he moved back in?
And was he sat down and briefed on these boundaries?
If you handle this properly right now, you may be able to spare yourselves and your son years of pain. Even if your boy never gets it, you will be in a much better place.
If you decide to play warden and try to manage his addiction/recovery, you very well may become addicted to his addiction like so many of us were/are.
Good luck.