My husband has a spot on the bottom of his foot that looks like a callous but it has a whole in the middle of it with a black drainage that has a bad odor?

See Podiatrist: Sounds like an ulcer with infection. Whatever it actual is, it is not normal and needs treatment.

Do not fool around: He needs to see a foot doctor asap. In the event your husband is diabetic go straight to the emergency room.

Necrotic: Wound. May be infected. I suggest he see a dr. As soon as possible.

Many possibilites: Some causes of foot ulcer include trauma, poor arterial or venous circulation, and skin cancer. Have it evaluated and treated appropriately.