I'm drowning in all his stuff

If he agrees with you in theory that he has too much stuff, then the next step is to nudge him into agreeing in practice.
Bite-sized chunks.
Set a particular wardrobe or cupboard.
Set a time.
‘On Monday I’m going to go through your wardrobe in the spare bedroom and take anything you don’t wear to the tip. Do you want to help?’
That way, going through the wardrobe is non-negotiable. The only thing he has to decide is whether to help you or not.

I’m assuming of course that you yourself don’t have any clothes you’ve not worn for several years, no photograph albums you never look at and no books you haven’t read in 40 years

I can but say a drastic down sizing from a 4 bed/3 bath/3 reception house standing in an acre in the country to a 2 bed/2 bath Retirement apartment was a case of must in our lives.
We both had been guilty hoarders and kept just about everything for all our years living here.

On our impending sale.............. a garage/garden sale and some 30 trips to the local "tip" got rid of most out door things that had been kept............auction house for furniture and antiques,local Antique Shops for less valuables, a local Charity shop sent a van to collect numerous black bags and enough coats/jackets and macs to start off a shop...................and hurrah!!!......... we could then just about see daylight again!!!

(But guess what?.............our sale fell through.......... .............and we are about to start all over again...............but without all the hoarding, because that has gone...............yippee!!!!!

Will he not agree to get rid of his video tapes? They are so out of date now that charity shops won't take them.
Can you go through his clothes with him? If he has, say, 10 business suits and is retired now, pick the best two suits for weddings and events and send the rest to charity or ebay?
As he agrees with you, would he be willing to do a bit at a time with you talking through what to keep or not?