I see. Well it’s understandable that you’re frustrated. But ultimately I think you have to put yourself first. Even if you plan on helping him. You can consider telling him you don’t want to be with him or talk with him when he’s high. Doesn’t seem like you want to hang out with him or like you can help him much when he’s in that state of mind.
Some info that might be relevant…
When he’s not high the weed will not make him paranoid or generally act like a high person (my experience). The high lasts about 1 hour but lingers for maybe 4. The long term effects are mainly dulling of cognitive abilities and inactivity/apathy (but my schizophrenia does this so much more than weed, you can’t even compare them).
If you want to help him, that’s great. If you could get him to stop smoking weed that would most likely also be great. But it affects everyone differently. It’s not necessarily worse than drinking even if you have sz. But for a lot of people (most?) it is. The best thing to do, of course, is normally to not use or abuse either weed, alcohol or harder drugs… or benzos, opiates, sleeping pills, etc.
You can help him when he’s not high if you want to. Distancing yourself from him probably won’t help him. But put yourself first, then help him if you can.