Sorry for your loss.
Just re read your post and its not only in the garage but in the car port too! I can see why that would make it difficult to sell the house!.
I know people like this and he's probably not doing it on purpose. He had a great idea about the business which he was quite motivated about - enough to collect allthe stuff in the first place - but then he couldn't get to the start point. This is probably why he hasn't managed to get his act together to move it.
It probably seems like such a massive job that he doesn't know where to start and daily life is so busy its tomorrow tomorrow. Also perhaps he hasn't yet relinquished his idea of the business.
However, you can't afford not to get the ball rolling, but I think if you want to save the relationship you have to make it realistic for him. It sounds like he's the kind of person with good intentions but needs prodding to get moving.
Would it help to break it into two parts and give two dates... One early date to move (and then clean) the car port as the most immediate obstacle.. and a second date to clear the garage. He might find it easier to contemplate doing it in stages and once he's done the car port, the garage may not seem as big a job because he'll have already started if you see what I mean.
Perhaps do a bit of real research on his behalf ... so that its not just saying out by x date... if he doesn't know where to start that might just mean he continues to do nothing ..
But if you pointed out to him that to move and put his stuff into storage would cost xyz... and that if he'd stored it instead of leaving it with you it would have cost him zyx so you have already saved him that much and allowed him to keep his options open but now you want your storage space back.
The trouble is selling something is always more time consuming than chucking it or moving it. So that is probably daunting for him too.
You can point out that having it sitting there and deteriorating is just losing value for him and he'd be better off getting some of the value back now.
Would it be possible to find out where buyers for that sort of thing are, and how much he could get for it.. or a quote for moving and storage...
At least that would give him a starting point and you'd soon see how co-operative he was if he responded to your helpful and researched suggestions.
It may be a pain but at least it would be realistically starting the process but you can also set a timeframe because everyone works better with a deadline. Also puts the focus on him sorting his stuff rather than you selling the house.