I think a 223 shooting TMKs, ELD-Ms, or Federal Fusions is a great option.
If the caliber make you, him, or his dad uncomfortable, than something 6mm, such as 6 ARC or 6 Creedmoor.
The 223 will let him shoot more due to cheaper ammo and much, much better barrel life than a 6 CM or a .243. Don't know how the barrel life is on the 6 ARC, but given its powder capacity and bore size it is probably not too bad.
If the caliber make you, him, or his dad uncomfortable, than something 6mm, such as 6 ARC or 6 Creedmoor.
The 223 will let him shoot more due to cheaper ammo and much, much better barrel life than a 6 CM or a .243. Don't know how the barrel life is on the 6 ARC, but given its powder capacity and bore size it is probably not too bad.