Pressure. Callouses develop when there is too much pressure and friction in the area of the foot. First par it down with a file. Then use spray deodorant (the armpit kind) to decrease sweat, this decreases friction. If that doesn't help, try otc inserts from a drug store to help off-load callus area. If fail, he may have a foot deformity or other pathology. Lastly, see a podiatrist. Hope that helps!
Corns and callouses. Can't be "ingorwn." they may feel painful and deep, but they don't grow "in." they are usually a symptom of a structural problem of the bones which cause rubbing on the floor or a shoe. The skin thickens as a protective response to the pressure. Do dad a favor and bring his feet to a podiatrist. You'll both feel much better afterwards!