Rot, infection or and burn. Hard to say from the pictures and my old eyes but I would see if you can put him on newspaper(off stuff that is gonna stick to it and embed in it) and treat the areas with neosporin CREAM...not ointment. Petroleum products can sometimes make it worse. See how it responds.
Be sure he has good ambient heat without too cold so he isn't sitting on too much heat and burning.
Be what I would do.
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Sonya
I'm not mean. You're just a sissy.
Happy Bunny

