It looks like a scabbed over wound. It doesn’t really look like fur mites generally do, the surrounding skin is so smooth and there’s no dandruff. Three things indicate that it could perhaps still be fur mites: That he hasn’t been bit by another bun, the location of the scab at the nape of the neck and the fact that the hair hasn’t grown back in 7 months.

If you scrape very gently on the scab with a q tip or the rounded back end of a plastic tooth stick, do you see skin flakes (dandruff like) then? Does the scab seem painful if you touch it?

Fur mites are easily remedied with Revolution (selamectin) or ivermectin. Both these antiparasitics are prescription only unless you’re in Australia.

Warning that we always issue when skin parasites are discussed: Never use any compound containing fipronil on rabbits. Fipronil is sold OTC under a variety of names, it’s safe for cats and dogs, but it’s deadly toxic to rabbits.

Are you in the US? It almost looks like a scab from an injection site.

Hmm, the only other thing I can think of then is an old wound where his hair was pulled out. Maybe one of the other buns did it. You said they don’t get along.

I would also agree it could be a scab from any needles. My bun had the same thing, I actually posted it on this forum a year ago before finding out it was necrosis. This picture looks just like my rabbit’s necrosis once the flappy part of the scab fell off (by itself) which took a week or so after noticing the patch.
Since your rabbit hasnt had any needles near him, then it is probably just from a wound.
Is there anything that he could be hitting his back on? Like a wood hut or what not?