I’m so sorry you have to go through this. The problem is, you’re right, they won’t asses him unless he’s off the drugs. Thats what the GP told me. You might want to try going private, if you can afford it. I think we were quoted about £400 for an assessment, but never went through with it, and I didn’t mention drugs at that point. With my loved one, I think it was the drugs that were making his paranoia worse. But even after being clean for a bit, he still believed everyone thought he was a horrible person. Though this might just be low self esteem now. Your son sounds like a more extreme case. I’m also sorry to say but it sounds like he should go rehab as a starting point. You can’t do this on your own, and if he doesn’t want help then it’s extremely hard to get it. He’d need to be a danger to himself to get into a hospital. Which tbh sounds like this is where he’s heading! I’d try 111 to start with, they might be able to guide you.
Good luck xx