What about setting aside a section of the garage/shed/whatever for the "useful" junk? When it's full, it's full. He can keep whatever he wants with no judgement, but it must be contained to that particular area. I can sort of empathize with his desire not to throw away things that are potentially useful; I was stuck in that mindset for years. But after realizing I just didn't have the time to fix said item, or that I was realistically never going to use it, I was able to make some changes.
Another thought... Could you maybe set a time limit on how long he has to decide what he's going to keep? DH and I have used that on each other. I'll get a box of his clutter together and tell him it has to be put away/trashed/donated within a week or I'll do it for him. He's done the same for me. It either provides the motivation to actually go through the items, or the realization that you really don't care what happens to it! He even took a set of dishes to Goodwill for me on the sly. I told him I wasn't going to use them, but I couldn't actually take them myself because they were a gift. One day, I just went down to the basement and they were gone. Obviously you shouldn't get rid of someone else's stuff without permission, as it could result in some very hurt feelings. But DH and I have a mutually-agreed upon system, and it seems to work for both of us.