Gee, it may be mites? But I have to be honest (not experienced), I would be making a vet visit. Not only to determine what the cause is and for it to be treated, but it also looks uncomfortable, I reckon a soothing cream of some sort may be required as well.
My boy sheds constantly, so I definitely get that…. and I noticed he had a few flakes too and discovered it was mites (plus fleas, good grief!), and even though Henry had a very small sore, it was nothing like I am seeing in your pic. I was able to treat Henry with puppy and kitten Revolution for the mites and fleas along with antiseptic cream for his little sore, and it did the trick – but your bunny’s sore looks way worse (sorry). I think he may need more care? Also, Revolution has to be purchased from a vet (outside of Australia), so if your there at the vet, you should get it checked properly and diagnosed. I am just guessing cause the symptoms sound familiar? Wishing you all the best cheeee. Let us know the outcome. Cheers.