You could use any of those three choices with your son and not be disappointed I believe. The tack I would take is to load the lighter cartridges right up where they are intended to run, or load the '06 with something a bit light (150 grains) and back off a bit to middle range loads with TSX, GMX, or Etip bullets. If your son is a bit on the light side, I think I'd let the rifle help decide which one matches his needs most - perhaps it doesn't matter- and then let him shoot plenty of practice with lighter loadings. A lot of younger and/or less experienced shooters suffer from recoil noise as much as the impulse effect.

About this time last year I was working on preparing my nine-year-old for potentially hunting moose. I had Ruger #1-B which I hoped would work and had some GMX loads ready. However, even with a shortened buttstock, that rifle was a bit too much for him to handle well - except on the bench. So I decided to let him try a Ruger #1-A in 270. We swapped the shortened butt over and gt him shooting 100 grainers until he was comfortable. I began to throw 140s loaded light next. I loaded a few A-Frames a bit heavier for hunting, made sure the rifle shot where it had at "moose distance" with the heavier loads, and we went hunting. It worked for us when he bagged a nice bull two weeks after his ten year birthday.

Sometimes, the air you 'let in'matters less than the air you 'let out'.