With reluctant husbands, I just focussed on my own stuff. I had tried to have him get rid of old t-shirts etc for YEARS. When I decided to declutter, I took Marie Kondo's advice and just did my own stuff.
Even though I'd been telling myself he was a huge part of the problem, actually, when I really really pared down my stuff, it made a huge difference. It was also amazing how much simpler our lives became, how much easier it was to clean and so on. DH couldn't help but see this, and, most importantly, see the huge improvement it brought.
Slowly (without my prompting, which would have made him get defensive, I think) he just started offering stuff up as I was decluttering. I made sure to have the 'donate' box clearly marked, and he just started putting stuff in it. The more we did, and the more our lives improved, the more he wanted to get rid of stuff.
So, lead by example. It's hard to keep your mouth shut, but, for me at least, it really worked.