Use the 6.5X55 with a good 120 to 140g. Have him practice. Yes a 243 can kill a moose if everything is just right, but given your options it is not the one I would pick. I don't think that the Swede will kick that much more than a 243 with 100 or 105g. At least to me they don't. It will be much less recoil than the 30.06 or the 303. The 6.5X55 is proven hundreds of times ever year on moose. My daughter loves my 6.5X57, she picked it over the wifes older 243 winchester.