Just because he's doing well in school doesn't mean he's exempt from addiction. Weed addiction can be a very big deal. This is especially true if it's a catalyst for depression/anxiety, prevents someone from finding a job, and drains a bank account. The stereotypical image of a burnout stoner is just a stereotype. I wore nice clothes, had a 3.6 GPA, and smoked 3-5 times a day.
If his gf walks out, it's his fault and he'll have to live with the devastation.
If he gets arrested with no money for fines or bail, that's his problem, don't rescue him. You can't pick his friends for him.
If he doesn't want a counselor, he doesn't want to change.
I know you're supportive of him continuing his education, especially at an accelerated pace. However, college is a privilege, not a right. The lack of respect and ungratefulness makes me feel like his sense of entitlement is through the roof. He seems to be holding his pace to graduate 3 semesters early over your head, and like he's using his increased workload as an excuse to not get a job or improve his life because he doesn't have the time for it. What's more important, him graduating early or fixing his messy life? Maybe it's time to ease up his schoolwork load so he has time for a job? If he can't find a job because he can't pass a drug test, that's his fault. What makes you think that'll change once he's graduated early and still smokes? Plenty of people in college have to work while they're in school and don't have the privilege of taking enough classes to graduate early, a privilege your son seems to be abusing.
I'd just firmly let him know that you're tired of his ungratefulness and lack of respect, and how he has become an emotional drain on you. Forget the whole weed thing when talking about it, he's made up his mind about it and it's his problem. I wouldn't throw him out of the house without warning, but let him know you're considering it. Might be the wake-up call he needs. Even if you did, you could still pay for college while he's on his own. He'd just have to find a way to pay for a place by himself, and that could very well mean cutting out the weed, finding a job, and subsequently graduating a little later.
Tough situation. Hang in there.