coloradojones,
I would think a young man your son's size could handle the 30/06 just fine. My own son, 18 yrs old, is really skinny....6' 2" and 130 lbs.! He has a Rem BDL in 30/06 (grad present) that gives him no trouble at all. I wouldn't be surprised if your son could learn to shoot the 7mm mag as long as he keeps it tight to the shoulder and takes his range sessions in small doses.
Have you ever considered the Sigarms San Swiss SHR970? It is a switch-barrel rifle of fine quality that can be had in a variety of magnum and standard caliber barrels. You buy one gun and add barrels (sometimes clips/mags too and/or bolts) to make up your different rigs. I'm not sure if the cost is prohibitive or not. You might be better off just grabbing 1 Savage rifle and adding others as time goes on. What hunter can stand having only 1 gun anyway? With your son's build I know he can handle the 30/06 and believe he could learn to shoot a 7mm mag with only a little instruction and practice. A shoulder pad like the ones P.A.S.T. makes would be very helpful at the range. I never seem to feel anything in the field.