Can you get a veterinarian involved? What is his temperature? Does the hoof/leg feel warm compared to his other ones?
Can you post more pics from different angles including one of the bottom of the hoof?
Maybe a severe case of mites. A vet would be ideal if you can. In the mean time, get his temp...i would give a shot of banamine if temp is not sub,, trim up his hooves and wash that area gently with a mix of vinegar and water. Be sure to check between his hoof toes. Pat dry well and apply nustock nice and thick. Re apply nustock at least 2 times a day. I would also give Ivomectin injectable at the rate of 1 cc per 40 sub q once a week for 3 weeks. He may need longer if not cleared fully. Make sure he has clean straw or pine shavings.