I don’t understand how bullets can be flying around, if he has a proper backstop? He should have a dirt or sand berm that is high enough to make sure nothing can exit the area. then he should have some wooden railroad ties in from of the berm for the bullets to enter. this is what we had growing up, and not once did a bullet exit the area.
If he is shooting at metal targets and the bullets are deflecting, then he needs to stop and change his target type.
When I was a kid, most .22 LR ammo was marked that the bullet could travel up to 1 mile.
480 yards is not far.
In this case it sounds like the berm is probably too low and of a shallow angle. Its deflecting the rounds up into the air rather than absorbing them.
Just my guess.