And it's not true that
Sapolsky's final chapter is War and Peace in which he looks at human capacity for actual and ritualised violence. Reflecting on the ubiquity of CCTV cameras, he cites an analysis of soccer hooligans frame-by-frame as they trade hoots and chest-beating with fans of The Other Side. Only a tiny fraction of the participants are even pretending to trade punches, the rest are surging about making a noise and sheltering behind each other as bottles and coins rain in from Over There.
If you want a brilliant example of the discrepancy between young men's capacity for pretend violence compared to meting out some actual blows, then get Rashomon out of your video closet. This early film by Akira "Seven Samurai" Kurosawa is a minute investigation of the nature of truth. An incident happens in a forest between a samurai, his wife and a bandit. The story is unpacked later in a ruined temple by the several witnesses. The samurai and the bandit have a fight in which both, according to their own "big me up" testimony, behaved with decision and courage. But an independent witness reports that the fighting cocks spent at least as much time trashing about the bushes with their eyes closed in terror as actually getting to get in some on-target hits. Chest-beating is all very well but if the other bloke has a sword, you might get hurt and we are programmed from young to avoid pain. [Exec Summary Rashomon in 9 mins].
Sapolsky cites On Killing: The Psychological Cost of Learning to Kill in War and Society by Dave Grossman for another telling example of having a cudgel but not taking it up. After the Battle of Gettysburg, 27,500 single-load muskets were recovered from the field. 24,000 = 90% of them had not been fired but still had powder and ball loaded. Half of these loaded guns had been loaded more than once and one had 23 musket balls choking up the barrel. In the fog of war, the poor squaddies couldn't remember if they'd loaded their gun, and in case would rather [re]load their weapon than actually point it at another man and pull the trigger. And most of the time they were just trying to get out of way of the incommming. Like in Rashomon, many of the heroes were made afterwards from the survivors recycling their own memories until they got to be able to live with them.
And what about PTSD, is that the cumulative load of being scared shitless day and night for months on end? Seemingly not: squaddies in the USAF drone patrol unit suffer as much mental damage as boys with guns deployed in Helmand Province in Afgo but more than medics on the ground in those dusty killing fields. The drone pilots are totally safe as they watch their computer screen in a bunker outside Reno, Nevada but they get to see the results of their actions bleeding out and turning cold in the night-vision camera shots on the VDU a foot from their eyes. Normal people are psychologically damaged by that experience.
In sum, neighbour, I don't think your son will have the chops to become a psycho-killer not matter what his score is on Call of Duty.