I was a string bean kid of 14 when I got my .270 but I had been shooting 12 gauge since age 10 (2 3/4 regular). The 270 was plenty at that age, but not too much, but then I had been shooting 22' s since I was 5, and a 410 since I was 7.

Assuming he has no prior experience with guns and he is " gearing up" for 2004' s deer hunt then, under supervision of course:
.223 first to master the basics, and several months later the
.308 when he is hungry (and ready) for it.

If he has experience already and is wanting to gear up for THIS deer season, then
.308, but supervise the " introduction process" .

EKM