This sounds really stressful for you all.
I think like some others have said, it's a misnomer to be trying to figure out what drugs he may or may not have been taking. His experiences might be related to drugs or they might not. That will all come out in time. We don't know as much as we would like about the link between skunk and psychosis, but these two things very often go together (not saying skunk use causes psychosis as we just don't know).
What is more important is the distress he's feeling now due to what sound like scary and vivid visual hallucinations and 'paranoia'. They might resolve if they are caused by an acute factor like drugs, or they might carry on regardless of what is causing them. In which case they may well be labelled psychosis.
I would also question if the lack of school achievement could be the early indicators of psychosis too (as people can be at risk for a while and become more isolated, start dropping out of things, etc, before they have the more well known experiences linked to psychosis, like voices and other hallucinations).
If this is psychosis, then the research is that the quicker he gets help, the better the outcomes. There are mental health teams (called Early Intervention services) dedicated to helping people in exactly that sort of situation. If you Google your local mental health trust, you can call them and get put through to your local EI team for advice. They will be able to guide you as to what the best next steps might be for your son and your local resources available.
Please don't leave it though and think oh it's just drugs, it'll all resolve itself. It might be a blip and he's fine today, but even if that is the case, he's still at an increased risk of developing psychosis with all the experiences you're describing x