We have downsized earlier this year. Large 3 bed with outbuildings and heaps of storage outside to a small 2 bedroomed bungalow.
We were ruthless in what we threw away, passed on, gave away for free and recycled.
Its a difficult process, throwing away things or giving away things that you have had for years so I get its difficult for him. But you can't hold on to the past and this a new chapter for you so he needs to look forwards and you will make it work.
We went through everything together, and put aside stuff to keep such as keepsakes and sentimental stuff and we bought the plastic lidded boxes for them. Everything else either went on the skip, if it was rubbish or on Facebook free sites and people came and picked the items up if they wanted them. I actually found it good giving away things to people who could use our unwanted furniture as they were stating out and didn't have much money, or people who could use things instead of us throwing them on a skip. You will be surprised with what people will take, and can use.
Once moved in, we have found places for things, and storage solutions that have been a game changer for us, that we couldn't see, or didn't think of until we moved in and we have an amazing boarded loft, ripe for conversion if we wish (but we won't) which we have utilised for storage. We have carpeted up there with spare carpet that we ripped up from the lounge and DH has organised all his things in boxes and everything is so easily accessible via the pull down loft ladder.
You will make it work and find a place for everything. But with downsizing you do have to be a bit brutal. All the stuff that fit into a 5 bed, won't fit into a 3.
But we love our new home. Less rooms to clean and vac, smaller garden which takes no time at all to maintain and mow and we are not spending our valuable time at weekends cleaning and mopping, weeding and mowing.
Our lives have changed for the better and we have not regretted the move for a second. We have decorated our new home and made some changes to make the spaces work for us and our dogs, with a guest room for when either of our adult DC want to come home and visit.