Grandtante that sounds eminently sensible for clothing. I'm sure that would get the pile down. And then when you both see what is left you can think of other ways to cut the amount down. Maybe if there are 2 almost identical suits - which is the nicest one, keep that, am assuming that while he will wear some sometimes it's not like he needs one.
every day like perhaps he might have in the past.
Marie Kondo suggests leaving sentimental stuff to the end of the process, which I think is very wise. What is sentimental will vary from person to person. I'm sure photos will be in this category
Videotapes - maybe first see if there are any duplicates and if there are ask him if you can get rid of those. Eventually, your video player will break and you can't get new ones so maybe he'd agree to look through them to decide which his favourites are, and replace them with secondhand DVDs, safely stored either out of their cases in suitable boxes or a folder. Any family videos then get someone to transfer them to DVDs, so you can enjoy them for years to come when your video player no longer works. My father's cousin took lots of movies on the old sort of film, but after she dies it was discovered that the films were no longer playable as the film had degraded. We'd have loved to have had the old family films transferred to a more modern medium.
Books, for me this would be sentimental in part. /might he be happy to pass on say beginner books about his hobbies on to someone just starting, and keep more advanced books, perhaps. Again are there duplicates, if so would he send one off to the charity shop? Any books on hobbies he no longer is really interested in, would he let these go so that someone else can benefit from them?
Would you feel better if they all fitted on the shelves? If so, work towards that.
DIY stuff - again are there any duplicates, would he pass on some to someone just starting off in life? Broken or rusty tools kept just in case, ask him if you can get rid of these. Decorating, do you have wallpaper from the decorating job before last, put on freecycle. Old paint - is it still usable? if it has gone hard then take to the tip. If it is still usable but an old colour, there are organisations that take usable paint for community projects or for people who cannot afford to pay for good quality paint - see if there is a community repaint near you. Or again freecycle
Not something Marie Kondo would agree with but I've come across the concept of an 'emotional withdrawal ' box, stuff you are not sure of, but don't want to get rid of just yet, put it away for a few months somewhere out of the way, and then decide if you've missed what is in the box. Chances are that most of the stuff will not be wanted, a few bits might, and that's ok.
Don't get rid of his things without his permission, if he is a true hoarder that will make things worse. It sounds as if you'll need to do a lot of the sorting for him, but it will be worth it to feel better. Make sure you keep on top of your stuff too, is there anything you could rehome. I don't know if this will apply but if he gets something new, try and help him find something old to get rid of, so things don't start to get worse.
It is not easy and it will take time, but I personally have found the more decluttering I do the easier it gets, and I have recently got rid of something that was expensive and of which I have fond memories using, as it would cost too much to make it workable again, and I'd not actually use it tbh, and it takes up so much room! I don't know if the perceived value of things is an issue, if something really is valuable then do try and sell it, but if it is just that it cost a lot in the first place, look at how much it might cost to replace as a second-hand article on eBay. If something isn't used, no matter how much it originally cost, it is not going to have cost any less sitting around at home.
there are lots of ideas for digitising music and photos, but thos will need to wait for another day,
Good luck, let us know how you get on and come back and share what worked for you