My uncle is a hoarder and after he moved into my grandmother's family-owned house (and filled it!) we got professional help, as he was no longer allowing most of the family into the house.
My mum contacted a professional organiser who works a lot with hoarders, and she put her in touch with a psychologist she often worked with. Their first tip for support people for the cleanup was to NEVER throw anything away without the hoarder's permission - it fuels the insecurity that is at the base of most hoarding, and you can guarantee that the person will end up replacing it, often with even more things.
It was incredibly hard work, as because he needed to be in charge and feel safe, it meant one session every two weeks and took the best part of the year - he also only allowed a few of us to help, as he was either embarrassed in front of some others or worried that they would throw too much out after previous experiences. It was worth it overall, though it was no quick fix and he does continue to accumulate things (apparently this is also very normal).
They way they worked was to have alternate weeks where he had a counselling session one week and a clearup the next. We made piles to keep, donate, sell, recycle and throw away - he was much happier to get rid of things if they were being donated and recycled, so we were pretty loose with the definitions of what could be used here (sorry to the recycling depots of our area!),
They added the rule that once he'd allocated to the piles he was not allowed to return and take things back, and at the beginning they didn't even allow him to touch items as they were allocated - he had to look and then tell the organiser/one of us, who moved it.
Hopefully some of these tactics might be useful - I would definitely recommend contacting someone to help, if that is financially possible and if he can agree. It took a lot of the personal strain out of a very tough time. Good luck 