Awww. Paula. I'm sorry that you feel disappointed in your sons choices.
The fact is he can do whatever he wants when he gets out. He is a grown man and he is free to make choices - good or bad. Maybe he hasn't learned his lesson yet. If he still thinks he can use and get by, then he might need some more "schooling" on the subject.
That is out of your control.
BUT you can control what you allow in your house. You can control who you allow to live there. If he wants to smoke pot, he can. If you don't support that lifestyle, then don't. You don't have to let him live under your roof. At least you know what his plans are now...
Have you looked into alanon or anything like that?