i am a bit like this.
as nox says, it may not be an OCD, it may well be a creative streak, my house is full of oddments, bits and pieces, material, clothes, props etc - but then that is my job, - i design film and tv sets etc - and if i remember rightly mrs nox is in a similar line of work, so its understandable.
my dad also owns a huge second hand shop, so its sort of a family trait in a way
i always see the potential in things that to others looks like junk.
i also know that the second i bin something, i will need it for a job or i will just discover a use for it.
i know it seems hard to understand to you, but as has been mentioned - its not your business.
my problem is not that theres too much stuff (though there is) its that there is nowhere decent to store it.
why not suggest to him that he gets some extra storage space? utilise high ceilings etc put shelves up etc
in terms of helping, I allowed my brother to help me sort some stuff out once, that i had left in my parents garage, and he started my just grabbing handfuls and throwing it in the back of a van to go to the tip - i had to tell him to go.
the problem with someone helping, is the fear that all they will do with everything you want to keep, they will sneer and roll their eyes and demand to know why you want to keep it etc, and they will not understand your reason no matter what it is.
if would suggest just leaving him alone until he asks but if you do want to help offer to help him construct some shelving or something, and offer to help put everything on the shelves or in boxes - don't try to bin things,
(of course if he is keeping stuff like garbage, and empty toothpaste tubes etc then its probably an OCD)