Once he gets 'unstuck' and back into the journey, he will be able to catch up with most, but maybe not all, that he missed. Getting back is the issue. You should be in consultation with a psychiatrist who is not primarily a medication psychiatrist, but someone who can take your son's age and circumstances into account and help you find the right pathway back.
I think if his classes right now are too hard for him due to this problem then he won't be motivated to keep trying. Rather than him failing the classes maybe he should take a semester or two off and focus on county college classes until he gets his confidence up. A friend didn't go to college right out of high school. Few year later when she decided that she needed a degree and knew what area to study so she went to college. She did very well in college since she had a goal in mind. She barely made it through high school. Not everyone one is reafy for college right out of high school. But only you and he know how well he is coping in his current situation and is addition tutors can help him through the semester or if he needs to take a break from a formal education for a bit.
It's interesting that you say "turns out" he was smoking pot in high school - it sounds like he was functioning well enough to graduate, and well enough that you didn't know he was smoking pot.
At 20, with a psychologist suggesting psychosis, it sounds like you need to be open to possible schizophrenia. That's the age, and those are the symptoms of onset. It may not be, but the path that you describe - heavy pot smoker in high school (but you weren't aware) not smoking so much in college but not manifesting symptoms a psych called psychosis, I think you need to be more open to different thoughts than marijuana use.
Best wishes for a better outcome than schizophrenia -