Plenty of hoarders here on AgTalk judging from the responses....  Exactly how bad is this?  Are we talking pilled to the ceiling, trails on the floor with no room to work at all?

The only way we got my dad's old shop cleaned out was to tear it down and build a new one. He didn't want to move all that old junk into a new shop, so it got pitched.

But it is EXTREMELY easy to get the infamous "shop amoeba" You set something down on the floor and it mysteriously grows and grow and eventually covers the entire floor so there are trails through all the junk and no place to work. It take a proactive approach to stay ahead of the shop amoeba.

The first step is being aware of its existence
the second step is to recognize it starting to happen early in its growth cycle(its easier to control when small)
the third is having somewhere the amoeba can grow and flouish to its hearts content...preferably somewhere outside of the main shop. For me its the hayloft in the old barn.

IMO, you shouldn't clean it out when he's gone. He should be part of the process since he created allowed the amoeba to grow.  Make it a winter project for the two of you.  It might be fun for him to see some of the stuff and try to remember what he was thinking when he put it into storage.  And you're not going to get rid of it all, not at first.  Which it might be helpful to have an old outbuilding where you can store this stuff.  Because if you do it without him it will just grow back in a few weeks/months, this will be an active project for your whole life.

Do you have a forklift?  get some old pallets and start putting some of that junk onto those pallets so it can be moved around a bit.  Get the tools organized and do what you can without actually throwing anything away.  Try and make a place to work, and if you have an old outbuilding not in use, move some of the junk that might have some value there.

Good luck, most hoarders are blind to their own clutter.  Start small, work your way into this.  Don't be judgemental when dealing with a hoarder...remember...baby steps..