He admitted he does have somewhat hoarder tendencies
While I have zero experience in his hobby, I have a lot of personal experience in the statement above. So I thought I'd respond a bit from my experience, to give you a little possibly insight into his brain in the hopes that it'll help you, and heck, maybe provide some therapy for me too.
It took me a long time to realize I had hoarding tendencies. In fact, it wasn't until I flipped to the hoarders tv show that I even knew it was a thing. But a light went on. Kind of like when I'd spent my entire life saving money, wanted to stop having to work as soon as possible, but didn't understand why no one else did. Then I found MMM and the light went on!
I have received zero therapy for it, so this is all my own self-discovery and self-diagnosis. From watching hoarders, it's usually at least one major event that triggers it. In my situation, the year after graduating college, my hometown suffered a major flood and every house in the town had damage. My parents also divorced when I was in high school and I moved away to go to college. But during high school and college, I kept my stuff in my bedroom in my mother's basement. My point is the basement flooded so I lost all of my stuff that was still at home.
As if that wasn't traumatic enough, I was the one who cleaned my mother's house and tossed out everything. So I spent time seeing the piles and piles of stuff on the curbs. Tons and tons of stuff going to the landfill.
From that point on, I know that I struggled to get rid of anything. Somewhat, because it was the last stuff I still had from my past. Somewhat, because I didn't want anything else to go in the trash if there was ANY chance someone else could use it. Thankfully recycling was getting more popular and with more items, so that helped me find a place to dispose of lots of things I'd held on for way too long (years of old magazines, college notebooks, etc.).
So my first advice would be to maybe find out what his traumatic event was. There may not be something, but there very likely may be. Helping to understand that, might help you understand what void he's filling.
Part two to my story, my parents divorce. Another event which I still don't think I realize how much it affected me. I didn't have good relationship role models and in fact, I just got married for the first time within the past year, in my 40s. I was a "great guy" and would help anyone with anything, loved being depended upon, to be the hero, to be able to help people out, and earn all of those great feelings. I think that was because I didn't get it as a child or in a healthy relationship. I was trying to earn it, and seek it out, no matter what it meant to me. So this could be part of his situation as well. In his past, he might have felt some isolation or loneliness, and now he has this new group of friends that depend on him, that he makes happy, and he enjoys it.
And in my case, combine those two things and you see the issue... here are all these tangible things that I made for people that they like, and I don't like to get rid of things. *boom* There it is! I'm going to hold onto these things forever.
Where I'm hopeful for him and you, as I mentioned I am recently married. I've been able to part with many more things, for many different reasons. One example, I grew up in the Star Wars era and had a decent collection of figurines. I couldn't part with those ever. They were one of the few childhood things that survived the flood. They were my favorite toys. However, all they did was sit in a cardboard box in my basement and collect dust. However, with the new movies coming out, my wife and her daughter agreed to watch the 6 movies with me. The daughter loved them. The wife thought they were just okay. But still, the three of us went to the most recent movie together. Guess what... I took those figurines out, showed them off once to them, cleaned them up, put them on Ebay, and was MORE than happy to sell them and get rid of them. I've never looked back.
I've also used Craigslist to sell many other things that I have for absolutely no reason to keep holding on to, but still had. I still struggle to throw stuff away. I love giving it to someone. I just really can't throw it out unless I know there is no use or no one wants it. So at least old magazines can be recycled, but old t-shirts are a perfect example of what I can't throw out... no matter how bad they get, I keep them to use as rags, but a person only needs so many rags! :) But I've since found companies that take old clothing and recycle them, or make them into rags. Perfect! I've unloaded a lot of clothing in rough shape.
My MMM knowledge has also helped me be more rational about the "cost" of storing these items even if it's just taking up space, or have been moved a couple times which I used to think was no big deal. But it's also helped me from accumulating anything new.
I still have way too many boxes. And it's still a struggle at times. Occasionally I get up the motivation to open a few boxes, get rid of a lot of things, and maybe end up with just one box, which is still progress.
Also, for sure having a good, healthy relationship, has definitely helped. She has been patient with me, and the more love I feel from her, the less I need some of these things from my past.
So I'll finish by saying I can totally relate to your significant other, and will caution you to tread lightly, but also give you hope that with patience, I think it will resolve itself. I agree with everyone to take interest in his costumes, or lay them out, take pictures, or whatever. That is probably all he needs is some type of scrapbook to go back and show them off, not the actual costume itself. He'd probably be fine selling them, especially after he sold one and realized the benefit of having the cash. He might even be fine donating them to a theater or museum, or a costume shop, or something where he could occasionally see someone else benefiting from them. Like how cool to have a kid come to his door for Halloween wearing one of his donated costumes.
I hope this insight has helped. Have patience and like an onion, slowly peel back the layers and see what you find. If you find a void, finding a way for his items to still fill that void, but in a different way, might be enough.
And finally, if you do finally get him to sell one. Don't push him right away for the second. Wait and see. Nothing would have been worse than having my wife just take a box and throw it out. I would have lost all trust. And any pushing to get rid of stuff only added to the stress. You might have to do all the investigation on your own, trying to find a buyer, and then flat out ask... "hey I found someone who would like to buy your _____ costume for _______. Are you interested?" and see what he says, and if he goes ahead, then slow down a bit and see how it affects him.
Good luck!!