Have your father checked for thyroid disease (probably not the case, but good to have it ruled out) . Does he drink enough water? Do the feet itch?
Itchy feet: athlete's feet. Antifungal creams twice daily for 4 weeks help.
If not fungus and merely dry feet:
Applying a skin exfoliant (salicylic acid pads) followed with vaseline petroleum jelly and socks help.
If it is just overly dry, cracked skin bag balm under occlusion will likely help. Have him apply a heavy coat and put his feet in plastic bags overnight. Sometimes chronic athlete's feet can be associated with the condition you are describing. Oral antifungals are generally the most helpful but you will need to see a doctor to get those