We just moved into a smaller place, today in fact. I have been decluttering and pressuring DH to do the same for over 3 months. I 'gave' him more that his fair share of storage space for his stuff - books, records, DVDs, CD's, old magazines, old electronics, random junk, etc. Then I radically pared my stuff down. I have been seriously annoyed that he has not done his fair share of getting rid of stuff and has not done his fair share of the moving/packing. He has had far more time than I to go through his stuff! BUT, I know he really isn't able to SEE that he has far too much stuff until he literally has to see it all. So tonight, I am on the internet because I have nothing left to unpack and organise and he is waist high in boxes of stuff wondering where it is all going to go. It has taken ages, but he's finally realised that he has to make hard choices and get rid of stuff. I know it will take him longer than it would me (he will want to list everything on Amazon/ebay to see if he can sell before he will donate) but at least he is doing it!
So I am wondering if your DH may have a similar issue - is he unable to see the urgency until your DS needs the room? If not, would setting a date for moving DS into the room be helpful?
I also second agreeing on deadlines and then you get to do what you want. DH is a serious procrastinator and I have started giving deadlines or there will literally be piles of stuff on the floor for YEARS. He'll grumble when I get rid, but can't get mad as he didn't hold up his end.
On the other hand, if it is seriously driving you nuts, you could just get rid and bear the wrath. If your DH gets over stuff like that I would go that route. My DH would hold a grudge and I would never hear the end, so not worth it for me.
Good luck anyway! I feel your pain!