As a reloader, I suspect that you know that this particular Sierra bullet is specifically designed for extremely rapid expansion, which is more suitable for smaller game such as varmints where penetration is not at a premium. No doubt, as you experienced, this bullet at close range on a smallish deer can be a killer for sure. I suspect that it more or less exploded upon impact.
Don't get me wrong, I am not knocking the kill ... and for sure I congradulate your young hunter on making a good shot. However if you'll take some advice from personal experience, I think over the long run you will be better served using a 100 gr. hunting bullet that is specifically designed for medium sized game such as white tailed deer .... such as Sierra's Game King, a Hornady SST, a Nosler Partion, etc. This will give your son a superb deer rifle set up for distances out to 200-300 yards ... and still be very effective at close distances. The added 15 gr. of bullet weight will not result in any appreciable increase in recoil.
Good luck and keep that son in the woods !! It'll do him good.