I'm not saying he used weed that night. I really can't imagine that. Even when psychotic, you can still make some risk assessment and if it's not mania but persecutory/paranoid, it's really very unlikely.

Delusions can resurface when you find new creative rationalisations after some doubt. You start to fear and the delusions will get stronger again. That could be it.

If he did use any substance that induced or reinvigorated a brief psychosis, it could be, if it's not too extreme, that he kept it for himself. If nobody can be trusted, you better not tell them you know.

Then later if you start to doubt your previous convictions a little or you think you can bring up the courage to confront, you may be more open about it.

It's all rather paradoxical maybe, but quite logical.

Someone in a severe state of fear either freezes or (depending on character) gets aggressive. Just like a panic attack only sometimes less visible. Unless it's aggressive, of course. You put more effort in to appear normal. Answer questions and so forth. Of course it is all a bit strange, detached.

I see it quite clearly, the being consumed by delusions (and/or hallucinations), but I honestly don't know if you would if you don't share the same kind of experience.

Trembling might be a good indicator. It is for me. Unless I am on antipsychotics, I always tremble (ironically ). In the midst of more severe psychosis, this gets far worse.

But detachment and seeming distractedness are also good indicators, though "just" intrusive thoughts can do the same. If he doesn't have a history of obsessions though, it's unlikely. There is a difference though, but again it's just recognition rather than reasoning.

Maybe this helps to clear up some things about his behaviour. I hope so. Maybe not.

At any rate, you do well, as others have said, to support him and see what may.



Edit:
I just read you said it also made him euphoric. Then it may be more likely he tried it of course.

It will pass, I'm sure. And he really shouldn't continue which may be problematic if he gets manic. But he seems to be quite open about it so that's good.