I don't know if you have siblings or if your father has siblings or a lawyer, etc. Could be he's made arrangements with them. Generally the person asks whoever they want as executor of their estate whether or not they'll serve when they make a will up. I would just ask him if he has a lawyer and/or if he's made any arrangements you need to know about, see what he says to that.

My husband and I are in the process of making up our wills now, have visited the lawyer once and go back next week. But we've only discussed it with the one of his three sons who will be the backup/final executor.