Why not just have him practice with a .22LR? A .22LR with a peep sight or low power scope and a brick of ammo will give him far more practice for the cost than any reduced load in a magnum will.

If he practices with the .22 and becomes a competent rifleman, he'll shoot his .270 WSM just fine. And he'll never feel the recoil when he has a deer in his crosshairs.

Mike