Maybe it’s just a male thing, but as much as we may appreciate a mother’s gentle love, we also thrive with the tough love found in masculine societies like in the military. Part of that is aggressive confrontation when we make errors that brings about discipline and eventually mutual respect that is earned through competence. You may see it as a verbal assault for no reason, that he’s being mistreated, but it’s actually a valuable, and in the case of the military, critical, part of male social interaction. Girls talk about their feelings (or just stab each other in the back). Boys fight. They have a direct confrontation and resolve the matter. As a man I would so much rather be screamed at by another man if I make a mistake, and find myself forced to do better. It makes a man rise to the occasion. Remember what your son is being trained for. This is serious business and mistakes, or even hesitation when given an order, can cost lives. I’m sure if your son actually volunteered to join the military, and particularly the US Marines with its special reputation, he made the decision seriously, knowing he’ll be shouted at and called names. He’ll be just fine, and come out of the experience stronger and more confident for it. He may forget, and question his choices in the middle of the experience, but that doesn’t mean he didn’t prepare as best he can for it, and will look back with pride by the end.
Perhaps, as a devout Catholic, you can consider the way in which God imposed discipline and justice in the Old Testament, and only after being strict (but fair) for many generations did He show He is a loving and forgiving God, rendering those early examples of justice all the more meaningful. Love and forgiveness is meaningless without first being strict. Your son will need a lot of discipline and harsh (but fair) justice when he starts his military training, and in the end he will earn the respect and admiration of his peers.