Welcome, EM. Sorry you had to find us but glad you did.
It's so hard to separate out what's "normal" sulky young adult stuff, what's experimentation, what's heading toward trouble.
I mean, even the perfect child 19-year-olds can be pretty jerky sometimes! Cleaving from our parents isn't always done gracefully. And 19 years old is still so young. Not an easy time at all, for them or for us. At least your son is working, and at least talking about future goals.
But disrespect is never okay, constant conflict is never okay, demanding your help on one hand while telling you to *$&^ off on the other is never okay. It's not fair to you, and I'm sure it's very hard for your other child to witness that sort of thing.
Pot, while not physically addicting, can still cause some dramatic mood changes and a serious lack of motivation, especially if it's done to excess.
My son started with pot and "fake" legal pot. In retrospect, I believe "fake" K2-type legal pot at a young age may even have caused some chemical changes in my son's brain that are just now beginning to be unraveled.
I'm not a prude about pot at all, but in his case pot and alcohol and a brain that was genetically inclined toward addiction led him to stronger drugs fast. Given your son's genetic history with addiction, I certainly understand your concern.
Do you and your son have a good baseline of communication, other than recently? Does it do any good to voice your concerns to him?
I think it's great you will be starting again with your counselor. My counselor and this place really help to anchor me when I feel like I'm going to be swept up again.