Hello everyone.
I wrote a big long post last night and Mumsnet swallowed it and I was so exhausted I gave up.
I have been off to see my dparents so they could meet Twilight and then for a romp in the woods with the dogs. I also went to the local pet shop and got her a new bed and some dog food. She seems to like the new bed as she has flaked out on it already 
I have A LOT to do as Mon and Tues have been extremely minimal. Our cupboards are full to bursting but the dcs start mid term tomorrow so I need to get as much done as I can today so I can play 
I am off to start by hanging a wash and rescuing the kitchen.
Scout I really feel for you. My dparents still have my toys from when I was a dc and I am 39. My ddad was so worried they would get broken that he wouldn't let dd play with some of them. Who exactly they were being preserved for I have no idea. He has sort of seen the madness of his ways but struggles with it.
That said, it dawned on my ddad that when he shuffles off this mortal coil it is my dbro and myself who will have the task of clearing his house so he has, over the last few years, made a massive effort to empty the loft etc.
I think for my dparents is brewed from being babies born at the very end of the war and therefore having so little as children. They then didn't have much money when we were wee so all our toys were hard earned and therefore really precious. They hate waste and it feels like waste to them to throw things out.
I have some of the same traits and ebaying things seems to help as I don't feel they are being wasted so much. Charity shops and giving to friends also helps.
The papers and magazines is something we suffer from in this house too. Dh is a member of many professional bodies and gets all their journals and also golf and running magazines. He can't see that this is clutter as he is not going to go back and reread them, yet my Weight Watcher magazines, which I keep for recipes, are clutter in his eyes. 
Everything moved from my dparents loft into ours is just clutter but the box of notes from when he was at Uni moved from his dmum's loft into ours is precious 
I have battled my hoarding and try harder and harder to get rid of things. I have had to do it the way bessie mentioned yesterday - going round and round a pile whittling it down. I find it hard to just chuck the lot, even though somewhere in me I know I won't use/reread/listen to the stuff again. The more I do it the better I get though.
Your idea sounds a good one. Keep one box down and get him to do, as zoo says, a small declutter every day. You can reward him with a
or choc biscuit or in any other way you choose
I know I respond well to praise and encouragement even at my age.