I have a partner who likes to keep EVERYTHING (I call this side of him Justin as in 'just in case') and is very resistant to change. He does have valid things-that-happened-in-the-past reasons for some of this, but even so, it's maddening.
I have been slowly decluttering our home of 20+ years for several years now, and he's always been OK with me doing it so long as 'his' stuff isn't being touched (though he's sometimes been quite critical). He remembers the cost of everything, sees throwing away anything even remotely potentially useful as a waste, and has really strong emotional attachments to things.
My approach has been to try to coax him along by leading by example, but I'm also very vocal about my decision-making process, how much I feel the difference in my mental health with an uncluttered environment, and how much I feel it 'costs' me to have so much space in my head taken up by navigating excess things.
There have been a few conversations about what we would want/need to take with us if we ever wanted to relocate overseas (not that this is actually likely to happen) or downsize (which I think may have sparked his imagination a little even though that would likely be at least 10 years away, unless our hand was forced by something drastic).
For a long, long time I felt like I was shouting into the void. It's also been abundantly clear to me that forcing the issue would never work with him and I would have to sit it out and wait for him to want to make changes himself (it has to be his idea, not mine!), and then go at his own pace (often weeks/months between voicing the intent to clear a space and actually starting to do it).
Over the last year he's been starting to let go of more without prompting. tygertygertyger's post about putting all of the duplicates in one place has definitely been a useful tactic (recently we've disposed of several bin bags full of old leads, power supplies, dead computer parts etc.), and now that we're preparing to have the house rewired, the practical benefits of having less stuff are starting to show themselves a lot more. There has been an almost seismic shift in activity (with him proactively sending me descriptions of things that he wants us to put on eBay so that all I have to do is snap a photo, copy the text and list them).
Good luck - it is really difficult.