This is particularly tricky because it's not your joint accumulation of crap over the years, so I'm not surprised he feels defensive about it. Having someone move into your space (even though invited and wanted!) would be difficult, at least for me. That said, moving INTO someone's space and having to live with all their crap would be doubly challenging. I don't suppose moving into new "our" space would be an option? (I know it's complicated by they're being a child.)
Anyway, a few years ago, DH and I cleaned our attic. The task was daunting and overwhelming and left to my own devices, I never would have done it. He kept saying, "We have to clean the attic!" and I kept saying, "No, we don't. If our kids have to throw it all away and curse us while doing it, that's fine. We don't have to do it." He is, I believe, borderline OCD, and I am, by nature, disorganized and scatterbrained and sentimental and easily-overwhelmed, as long as it's out of sight, I'm fine with mess.
He promised to make it very easy for me, and also promised that I would be the one making all the decisions about my own stuff. I sat in a comfy chair, while he brought things downstairs, grouped like with like, I made very quick decisions about whether to keep, donate, throw away, etc., and he whisked it away to the appropriate place, and brought me more stuff. That way, there was always a big enough pile of things on front of me to motivate me to make quick decisions (couldn't afford to go down the rabbit hole that is memory lane, which is what I normally do if sorting myself). But he was careful not to pile so much in front of me that I was overwhelmed.
We actually took two days off work to do this. He didn't argue with any of my decisions, and the fact that he was doing all the moving of items, organizing, and putting them back away, kept me from feeling overwhelmed by the task at hand. It worked beautifully. In the end, I got rid of about 90% of the stuff that was in the attic, and was able to do it easily because of how we went about it.
Also, I set some guidelines for myself and decide that something had to be two out of these three things in order for me to keep it: beautiful, sentimental, functional. If it was only one of those things, I generally got rid of it (a few exceptions for truly sentimental things, but only a few, because even if it's sentimental, but I'm not going to use it or display it, what's the point? Take a picture and move on.)
So, not sure if there's anything useful in there for you, but that approach worked well for us.