Is any of the stuff ok to give to a charity shop instead of going to the tip? That might be a bit easier for him as it's not being binned and someone else can buy it and use it.
My mission is to sort our house by the end of the year and I am getting there slowly. I have to say today we are sorting out x cupboard or getting down x boxes from the loft and we are not having a nice family day until it is done. Then if I say "do you want to keep this" or "can I throw this away" a lot gets kept. Instead I say "this has been up the loft for years so we'll get rid of it". If he has to actively disagree and say "actually no, I want to keep it" a lot less gets kept.
Unless you are really lacking in storage space I wouldn't add some - agree with others it will just add to the stuff everywhere.
The thing that drives me mad is the bags and bags of rubbish - receipts, crisp packets, 1ps etc that I find stuffed in every cupboard. At least they can be chucked (and I keep all the money I find, which really adds up) but it drives me mad.
If he is really incapable of going to the tip is it worth just putting any bags he says are for the tip in a cupboard/garage so he can't see them and change his mind, then take them to the tip yourself when he isn't around?
I do also chuck some stuff out without asking which I know isn't a good idea.
Also what about your stuff? I am a real hoarder when it comes to books. When I started getting rid of stuff I went through them and got rid of about half - they now fit on the bookshelves! It also helped as I can say when going through stuff that I got rid of hundreds of books so it isn't just his stuff that is going.