There's much to be said for stocks with adjustable lengths of pull. From a young man's teenage years until his early 20s height increases, sometimes dramatically in short periods of time, but from 20 to 30 bulk may increase through his chest, arms, and shoulders which actually reduces the appropriate LOP. Differences in seasonal clothing also affects LOP as anyone who shoots in both the heat of summer and the cold of winter will understand, so a stock with an adjustable LOP has year round utility even after someone stops growing. IMHO, there is nothing your son will be able to do with a full size .308 that he can't already do with the carbine, except get puffed out more quickly if hunting in rough or steep country. A 20" barrel generally seems to suit the .308, a 24" barrel seems to suit the .30/06, and a 26" barrel seems to suit the .300 Winchester.