I disagree with those that think it's his junk - don't touch or throw . . . .

Hoarder's are sick and need help . . . He know's you are cleaning organizing - so go at it.

As a few mentioned, pallets or the big totes (caged plastic deal) - put questionable stuff on/in them to decide to throw later. Throw all trash/unusable junk .. . . .

I had to help clean and organize a shop . . . . the person responsible for the shop wasn't happy, he got over it and good stuff was sold on a online auction and garbage trucks of trash was land filled and burned (although, he does throw it up once in a while). In this case, If he doesn't want to take responsibility for keeping it clean and organized, he gets no say in what to keep and what to throw . . .. .  (little different then your situation).

Clean away and organize, organize and keep it organized . . . . .. . Cleaning and organizing the farm shop is no different task then harvesting, planting, field work and the dozen's of other farm tasks that need done.

Edit: So now, that I pee'd all over others door steps . . . I better go do some organizing and floor cleaning in my own shop ! I only wish someone would come help or clean for me.