I am a confirmed .22RF guy, yet find myself checking out these posts on the .17RFs. Im starting to see some sense in the M2 for accurate 'short range' small varmint control where winging off a bounced .22 might be a bad thing. Seems the .17 tend to disintegrate about where they land.
Yet, for me, the truth is the .22s are the basis for all other shooting, and the levels of performance available today in that chambering is more than enough. If it exceeds 75ish yds I can go to the .22WMR.
Your young fellow will be well served with a good .22LR, and afford to shoot it a lot. Not so economical with the .17s, and there is no substitute for lots of shooting.