I talked to now axh mom about his drinking. It didn't get me anywhere. Her comments are "everyone smokes pot".... blah blah. Her husband was an alcoholic so she was a typical codie.
Did it help me, no not really. But if you feel they will support you and you feel they need to know, that is your decision. If you feel they will support you and get angry at their son, I don't really think that will happen. They will enable him more, as we all try and "help" our addict. Good luck with your decision.