Dad now lives with us, and when he moved here he brought his favorite stuff. Some of it still stored in our carriage house. However as far has worsened, instead of his favorite chair he needed (and we bought him) a lift chair with ottoman. Also his bed was a queen, now he needs (and we bought him) a medical adjustable bed. It occurs to me that we have spent our money to provide what he needed-- can we safely get rid of (donate) his old stuff? Not much room to store the old.
I would sooner get rid of the bed and chair than papers or nicknacks. You could tell him they just got moldy. What I'm doing with my husband's computer magazines is to throw them out a year at a time, but I will always keep one year's worth, and tell him the others are in the cellar, and I'll get them tomorrow.
Cure for cancer? How about a cure for hoarding?
I say GET RID OF IT! It's your house, so tell him you just don't have the room to store it any longer! Maybe some of it can go to otheres in need, but it's outdated, old, and possibly stinky! Its easy enough to get new, even if you have to buy off of Craigslist, or second hand, and probably No One will want it either!
If he ends up Ever moving out on his own, probably the most he will need can be wrangled out of your home anyways, as he is likely to go to a Senior Living Apartment, Assisted living, or a Nursing home, all of which are generally very small and can't hold a lot. Spare yourself the aggravation, and get rid of it, I mean Donate it all to someone deserving! 😉
Also, assuming you continue to store these items indefinitely, what does he want you to do with them after he dies?
Questions like that might help him think this through better.
I have to say that I have a very hard time parting with things. I'd probably tell you the same thing -- hang on to the stuff, please. But I think I could be guided to think more rationally! (I hope.)
Another thought is to get a brochure from a storage place and show him what storing these possessions would cost per month.