You didn't say where you live but if they are cracking like that it sounds like they are dry. Get him a mud hole to walk in. Flood the water trough so he has to walk in mud to drink. Pull his shoes and trim him. And if it were me I would go to useing An easy boot or something like it to ride in for a few trimmings till his moistend feet grew out. I don't shoe my mules down here on the desert but when we go into rocky country I have a pair of Boa Boots I use. When shoers cost $150.00 a shot they pay for themselves real fast. But get some moisture on those feet.