AmericanGirl gives good advice.
I would start by mining his school's website for anyone who can help you navigate these difficult waters. My difficult child's school was less than helpful, but my PC18s school has many resources for parents in your (our) shoes. Had I known that difficult child's school would complicate things, I might have reached out to someone in the student wellness department or even at their Newman community (we're Catholic) there instead of the Dean. Or I would have contacted a ATOD counselor in the college town. I would get as much professional advice as you can. Seek a counselor in your area to help you deal with the fallout and to help your son. Call your son's pediatrician or family doctor and ask for advice. Even his HS guidance counselor or school social worker. In retrospect, I think if we had confronted our difficult child in a counselor's office it may have gone better. I also think we needed options, we had now idea how he might react and it never dawned on us that he would storm out. It caught us completely off guard. I found this board that very night, because I was so shell shocked and had no idea what I should do next.
I was in your very shoes and I blew it. My son started using drugs, drinking in college. He went from being a straight A HS student who was very close to us to being a C-, unmotivated kid on the slide, who lied about everything. His first night home after freshman year, he came home stoned. He was insolent and secretive all summer, culminating in him renting an off campus apartment for his sophomore year without our knowledge. We tried to go with the flow, chalked it up to normal growing pains. The day he was to go back to school, I spied an internet order of a large quantity of marijuana paraphernalia and we confronted him and told him we could not support sending him back to school. He left anyway-completely shocking us - and we were estranged for over a year. He subsequently failed out of school and the bitter, 18 month estrangement was the hardest and most heartbreaking time in my life. Things are better now, but still not terrific, he is 21 and back home, a college dropout with limited options and I am not sure where our relationship will go or how his future will pan out. In a lot of ways, confronting him threw him from the frying pan into the fire. I had no idea it would end that way. Now, I am not sure that any other approach would have worked better, but now is the time to get your ducks in a row.
Keep posting...
Edited to add * also, if you have custodial accounts - bonds, childhood savings accounts etc, in your name or his name, now is the time to empty them and stow the money for safe keeping. My son liquidated the savings account we had started for him as a child, and it fueled his misbehavior and his separation from us. Sneaky, but also consider putting a credit alert on his credit history as though you are him. Yes, it's illegal to assume his identity for those reasons, but it will slow down his ability to get instant credit and all is fair at this point. The Dean of Students at my difficult child's school pointed him towards the Credit Union and a student Visa credit card to fund his independence... ugh.