Maybe talk him into doing a yard sell? Then donate or take to the dump anything that is left.
Maybe ask him not to collect more and focus on what he currently has until he recoops a certain amount of money. Or ask that he only use a designated space and he can fit as much as he wants in that space but what doesn't fit has to go. Tell him you're going to garden and argue it as saving money on produce. My dad was this person and the only way my mom ever got him to clean up his junk was to give him options as compromise that used his logic against him or an ultimatum.
My man is like this, he has severe adhd. I don’t have to deal with it though because that shit is at his own house. Same excuses, he can make money. I go over there and start listing the stuff for sale. He can’t get mad right? It’s to make money. Which reminds me, If any one needs 150 bathroom safety railings brand new lmk im selling them. LOL.
My partner keeps boxes and boxes of empty cans and old newspapers! Wtf? I just throw them away when he isn’t here 🤣
I grew up with a hoarder so I'm pretty minimalist. I just periodically throw things out that haven't been touched in over a year. My husband usually buys practical stuff (usually) but I'm still always de-cluttering and I do discourage a lot of things- at least until other things are organized/ donated/used / put in storage etc. At least I know it bothers me more than him. If it was ME creating clutter that bothered HIM I'd be more stressed but the state of our house mostly depends on how I feel, he's pretty indifferent about it.
It's a psychological thing. I would get rid of one thing a week, casually. Lol Sorry you're going through that.