So I have good news and bad news. The good news is he's trying to learn about clothing already, so he's already on the right path. The bad news is
>>128787 has given you terrible advice, because using trickery, subterfuge and ultimatums to influence your loved ones is a very silly idea. He'll only resent you for forcing him out of his comfort zone.
You could try finding some mensware bloggers, writers, YouTubers, etc. so he can educate himself further. Also it's worth remembering that the men's section of any highstreet clothes retailer is full of badly fitted, uninspiring, and either way-too-on-trend or years behind items. This is why today you can see public wonders, like 40 year old men wearing skinny jeans, or whatever the hell pic related is. He has probably never had any guidence on how to not dress like a 16 year old, and if he tried bringing it up around other men, there's a high chance they'd just call him gay. He's basically learning Chinese without ever having so much as overheard a Chinese person actually speak the language, so it's going to take him a while before he doesn't look like Chris Chan at a disco.
Finally, this sounds really dumb, and I am both that and tired, but watch some earlier Michael Mann films with him. Every man in those films is very well dressed*, and even though the clothes are 30-40 years old, any man wearing them would only look out of place today because his contemporaries are wearing polo shirts with low-rise drain pipe trousers. You could actually employ some trickery here and casually comment on how well dressed the likes of Will Graham are.
*This doesn't apply to The Last of the Mohicans or The Keep, fyi.
