I think that if you switched to the 240gr. packs of XTP's that you find in the stores that come with sabots, you would decrease the recoil a lot. That, or switch to pyrodex powder. I've found that heavy conicals, and Triple 7 both hit my shoulders harder than sabots, or other types of powder. Another approach to cutting recoil would be to switch to a hard cast bullet with a big meplat on it, and go down to as low as 50gr. of powder and still knock the stuffing out of deer. Those level loads are used on deer out of pistols(1000-1200fps), and penetrate completely, I don't see why it wouldn't work the same out of a muzzleloader. That should really cut the recoil down for your son, hope that helps.
~Amos