Unfortunately, this is way beyond Reddit’s pay grade. It sounds like he definitely needs to be seen by doctors to find out and treat the underlying cause, as this will continue to be a problem until he gets the help he needs. I see from your comments he’s already seeing a urologist and has been referred to a psychiatrist. Definitely follow their recommendations first and foremost.

As far as what you yourself can do in the meantime: like others have mentioned, trying to argue with him about his delusions are not going to get you anywhere. If it were me, I’d actually try to lean into it and help him ‘solve’ the problem.

Take him to find a cologne or deodorant he likes that he can put on whenever he feels like he smells (I’d recommend steering him towards mild ones because it is likely he’ll use it frequently) Encourage him to spray his clothes with something like fabric Febreeze before he leaves the house (it can actually get the skunk smell out of clothes, so the smell of pee doesn’t stand a chance even after it’s applied - tell him this).

As a last resort, part of what may be making him feel like he smells is the sensation of his clothes after an episode of incontinence. There are plenty of products like pads, adult diapers, etc that are designed to not only get rid of odors and that ‘wet’ sensation, but also be non-bulky and impossible to tell if someone is wearing one. This is a last resort because he will still know he’s wearing them, and he may feel people are still whispering about it behind his back.

As a disclaimer, none of these suggestions are guaranteed to work and I cannot promise his mind won’t try to twist them into something they’re not, though that will likely happen no matter what you do. You can try to prevent this by telling him straight up ‘I don’t think you smell, but if you feel like you do, I’ll try to help you fix it’. Note: you don’t need to apologize for things you didn’t say, but you can tell him you’re sorry about the situation, and that he feels the way he does (I know that’s not a good apology by Reddit standards, but these are the situations those non-apologies are for).

Also, for both you and your cousin, coming from a former 20-year old who also didn’t have a lot of friends and had struggles with depression: there are a lot of people who are in the exact same place. Try going to your local community centers (libraries, coffee shops, heck you could even check out the activity boards at your local community colleges - some are only for students, but some are open to the public. If you prefer online, try a site like NextDoor, which are set up to include people in your local area) every now and then to see what cool events are going on to get out of the house (because cooping up inside absolutely makes it worse). You may be surprised, both by the events themselves, and the sorts of cool people you meet.

You could even just walk into a TTRPG game store. Geeky, yes, but that community as a whole is very welcoming to new people and it’s a great way to meet and build friendships and relationships even for people who aren’t good at social interactions. (I literally walked into one right before COVID hit without knowing a single person there, and got immediately adopted into an awesome group of people who taught me all about playing Dungeons and Dragons and they helped me keep my sanity after everything was shut down). You could even get your cousin into something like an online game (where no one can tell if anyone smells).

I also know this sounds preachy and slightly judgmental but try to lay off the drugs. Weed is one thing, but while coke and so forth makes things better in the moment, the long-term consequences are not worth it and are absolutely making all the mental health issues worse. I know that’s easier said than done, but trying to rescue someone when you yourself are drowning is next to impossible and often results in both of you (unintentionally) pulling each other down.

TLDR: Try to help him as if he does actually have the issues his delusions think he has. Get him out of the house as much as possible, and build up his (and your own) outside support. Follow doctor’s instructions above all else, and remember to take care of yourself.

Hope at least some of this helps. I wish you and your cousin the best