You know, I used to always standardize on the 150g for .30-30, because it was more common and shot flatter, and the light carbines didn't slap as hard with it.
But now that the 170's are more common than they used to be, and after having read Paul B's discussions on heavier bullets in the .30-30, and considering the shorter ranges that one will actually be using the handy iron-sighted rifles (which means the greater drop is less significant), I'm leaning toward the 170, nowadays.
If your grandson is of a size, and wears some padding when he practices so as not to develop flinch, I'd go with the 170.
Of course, you've already touched on the number-one issue, which is keeping your shots within a reasonable distance. At 75 yards, either one will do the job cleanly on any deer that ever lived, and if the boy is at all recoil sensitive, better to use the lighter bullet that he'll be more apt to practice with. One of the beauties of the .30-30, as far as I'm concerned, is the lower cost of factory ammunition to practice with, so that one can feel absolutely confident come time for the shot.
Will you, too, be one who stands in the gap?