Like Eric said, many states don't allow them for deer. If legal, I would not use anything less than a .44 Old Army (.457 RB, 456 RN) with a full load of 3f. And then a lot of practice- after which he might rethink the idea😀! .50, .54, .58 with a heavy load would do it, but might as well use a bow, it would be a very short range choice.
Hawkdancer