My rabbits both have sore hocks because they were on concrete floor. (Not anymore, of course.)
They appeared as angry red bald spots on the very corner of their heels, hardly visible without pushing the fur away. But I caught them early; if they worsened, they would eventually form crusty scabs, eventually cracking and bleeding (at which point they are life-threatening as infection sets in and abscesses form).
Ulcerative podermatitus, or sore hocks, occur because rabbits' feet are made to lie flat on cushy, loose ground, with their claws sunk into it. When put on wire flooring, the usual cause of sore hocks, the wires dig into their feet, causing sores. When put on hard flooring, a less known cause and the issue in my case, their claws end up propping up their toes, shifting their weight from being evenly distributed along their foot to being significantly on their heels. The unnatural wear and rubbing over time causes ulcers to form. Sore hocks can occasionally happen due to slippery floor, as well, though slippery floor tends to cause joint problems more than sore hocks.
From what you've said, it doesn't sound like their flooring is particularly bad. If they're outdoor rabbits running around on grass, that's literally the best possible flooring they could have. It also doesn't look like sore hocks to me in the pictures, because again, they're caused by unnecessary pressure and rubbing (think blisters, essentially). I don't think you would get sore hocks anywhere but the hocks (heels) unless they're standing on wire.
But, just to cover your bases... If it is sore hocks, you're pretty much out of luck. They're almost impossible to treat, because you can't exactly put your rabbit in a wheelchair to keep them off their feet. The only solution is to clean them daily with antibacterial solution, and improve the flooring. Mine got the sore hocks diagnosis in April, they've had better flooring for 10 months now, and the most I can say is that it hasn't gotten worse, which is actually an accomplishment. My vet says it's almost unheard of for rabbits to get rid of sore hocks once they have them, because it's just so hard to create the conditions necessary to prevent them from being further aggravated. I've put mountains of effort into constantly improving their enclosure; there are no hard surfaces, the plastic bin is lined with grass mats, the concrete is covered with foam exercise mats, the wooden box is topped with a folded towel... And after several months of them successfully not destroying a piece of polar fleece that was their trial carpet, they're now getting pile carpet on the floor, which is the best indoor equivalent of grass.
But I don't think it is sore hocks your bun has. Here's hoping its something more treatable!
Good luck! My rabbit has a small sore blister on his back hock, but I put some memory foam bath mats down in his cage and it seems to be good for him.
