Your son, is right, and being a "sniper" he should be able to explain this to you in better detail than my explanation.
Parallax is the error in apparent point of aim verses actual point of aim due to misalignment of the shooter's eye verses the scope's axis (line along the bore of the rifle). Scopes can be made to correct this problem. Scopes correct this by either having an adjustable or fixed parallax. Fixed parallax means the distance at which there is no "error" is fixed to something for example:
100 or 200 yards from the factory (who ever made the scope). Most "tactical scopes" have an adjustable parallax, which means the user can adjust the parallax error free distance on the fly to reduce parallax error whatever the current target's distance. Finding the parallax is important -- hitting a 4" target at 400 will seem impossible even if you have the correct dope dialed in if your parallax is off.
There are many factors besides geometry-- proper check weld, consistent ammo, wind conditions at target site, firing site, and everything in between-- weather conditions, heat from the barrel, the list goes on and on.
PM me if you want a more detailed explanation and how a scope corrects the problem plus the different conditions that effect shot placement. I'll get you pointed you in the right direction. You can take the info and impress "your one shot one kill" son