My advice - 45.70 Gov. But ... I have a Marlin Guide in 444 Marlin that is a great shooter. In my opinion, the recoil of both the 45.70 Gov. and 444 Marlin factory ammo will be tough for a young kid. As mentioned before if you can hand load, you can push the load down to a minimum safe charge and eliminate a good deal of the recoil. However the 44 Mag. will probably still be the way to go for your yound son. Just keep in mind that the 44 Mag. is no match at longer ranges for these other two cartridges.