If he's a "pack rat" - my term for people who aren't really hoarders but tend to accumulate - I'm sure some suggestions (like his own room or other space for his stuff) would work.
I've also come to the conclusion that hoarding (and the subsequent taking up of lots of space in the house) is a narcissistic behavior. And narcissism at any level is very hard to combat.
By this definition, my husband is a pack rat. It makes me a little nuts... I mean, it literally gives me anxiety to see clutter of the amount he is OK with. I have to remember that this is a learned behavior on both our parts. Growing up my mother was very, very strict about keeping the home clutter-free and clean, but has now moved on to hoarding behavior. Her mental processes about the home have never been healthy, I realize. Meanwhile my husband grew up in a house that can be kindly described as "not very tidy". Frankly, I can't stand to be inside his mother's house. I sit outside so I can breathe. It's dusty, it's cluttered, she smokes, there are pets, I'm always about 2 seconds away from screaming and running out like my hair is on fire when I'm there. I refuse to sleep there anymore. So he comes into our home and sees some crumbs on the kitchen floor, dishes on the counter, toys on the living room floor and the front porch not swept and he thinks the house looks "fine" and I am ready to cry because I'm completely certain CPS would take my daughter if they saw the state of the home. The whole issue keeps coming to a head, and I think we're working on a solution that looks roughly like this: The garage is his, but I get veto power. The inside is mine, but he gets veto power. (Because I would totally make it all pink and purple and bunnies and he's not having it.) We will have photos or a list that acts as "best practices and standards" and we are BOTH responsible for keeping the house in that state. The devil is now in the details. He's always amazed that the bathroom gets cleaned every week, I am always disgusted that the bathroom is only getting cleaned once a week.
But back to the stuff... having the garage lets him have some "stuff" that he can squirrel away and stroke and talk to lovingly (this is what I picture him doing with his "junk") and it allows me to not see it. There has to be a strict limit with someone who pack rats. I let him have the office too, and I am so, so, so sorry. He's gotten rid of a few boxes. Our dynamic is such that I just keep breakin' his balls about his junk, and he keeps working on not having so much junk. But the effort, I have noticed, has to be constant on my part. I nip clutter where I can see it. I make him throw cards away as soon as the holiday can be called "passed". If he says he'll use it later, I ask him "For what?" and if he has no ideas, I throw it away right there. He screams and howls, but at the end of the day he admits it helps him. The narcissism, that I think goes both ways. I demand a spotless house at all times, and the poor man has nowhere for his treasures! He wants to keep everything single that that might be useful ever, and doesn't seem to care that I am hyperventilating just looking at it. We're a unique study on the two sides of the "stuff" issue.