you can do.
your son is an adult now and has to make his own choices, no matter how bad they are. you can only hope that at some point the consequences of those choices reach a point where he's sick and tired of the life he's created for himself and wants to do something about his drug and alcohol problems.
so while you can make your feelings clear and what you think the best course of action would be to take, you can't force your son to take that course of action or for his father to agree with what you propose.
the hardest part as a parent is sometimes the act 