Thanks for taking the time to respond nonetheless. No, he is not going bald on the back. There is just obvious chunks of fur being shed, but there must be a lot of it growing in as well. It just looks very mottly. He is all black, but the fur in the back looks all messy, grey and white. (The undercoat colors showing)

They are fairly large chunks–one inch in diameter, but he still has a lot of fur back there nonetheless.  I guess it gets quickly replaced!

In terms of his diet, he eats better than me, haha.   He has hay all the time–the majority timothy hay with a small spattering of alfalfa.  He eats dark leaf vegetables (spinach, kale, bok choy, romaine lettuce, red and green chard, escarole, turnip greens–yes I am being thorough here I know.., yellow long beans, carrots, broccolli.  Each morning he gets a large plate of at least two different kinds of the aforementioned veggies and in the evening a small dishful of rabbit pellets.  I find completely feeding him veggies gives him runny buttons, so I give the pellets as the second meal.   Of course the hay is always present and he always eats it all.

I never see him grooming his bum–I don’t think he can reach that far, which could be why it is so mottled.

I don’t usually need to give him bum baths–I check about once a week to see if he needs anything cleaned, and of course he hates me for picking him up!

I also have a dehumidifier in the room he stays in.  I’ll use it a little more if you think humidity might be a possible cause.