I have used 3006, 7mm rem and 270win for elk & moose. Loaded with premium bullets such as Barnes TSX, Nosler Partitions or AB, TBBC, Aframe, etc they'll work just fine on swamp donkey. Of the 3 listed I prefer the 7mm rem mag.

If dad is thinking a new rifle is requiredsuggest he jumps to the 338wm and load it with 225 or 250 gr pills. Not required but certainly one you can't go wrong with either.