May you have peace. Truly. I hope what I am about to share doesn't offend, but it will be the truth:
First, contrary to the false teaching of the WTBTS, participating in military service is not wrong. John the Baptist, of whom Christ himself said "Among those born of women there has not been raised up greater than John the Baptist..." showed this when he addressed some in military service. In answer to their question as to what they should do in order to "produce fruits that befit repentance," he did not tell them to leave the service. Instead, he told them, "Do not harass anybody or accuse anybody falsely, but be satisfied with your provisions." (Luke 3:14) I give you that so that if your conscience is troubled because you beloved son IS in military service... and I am not saying that it is... you can know that it need not be. At least, not with regard to his enlistment and participation, per se.
There is more, however, and this is the rub: as my Lord also said, "He that lives by the sword... will die by the sword." What does that mean? Obviously, it is not a definite - many have participated in military service and not died as a result. What he meant is that anyone who wishes to live by the sword... assumes the RISKS that come with doing so. In your son's case, he chose to be a part of an institution whose primary purpose is to fight. True, the fight is supposed to be against the country's "enemies"... but it can and does very often turn out to be various sub-fights... between perceived sub-"enemies". And, as someone pointed out, there is a LOT of testosterone on a military installation... which very often needs some kind of outlet. That outlet usually comes in the form of lots of sex... or, alternatively, lots of fighting.
Your son chose to be a soldier... i.e., a fighter. And so, he got in a fight. Does it matter that it was with one person or 10? Does it matter that it was here... or there? If he can't take 10 guys jumping on him here, how in the world is he going to be able to take 10... if not more... capturing and/or maybe even torturing him "over there"?
He's young... and your "momminess" is coming out, as well it should. But... he made a choice. Your BEST "mommy" move now... is to let him live with that choice... as well as be allowed to make future choices, including what, if anything, to do about this incident. Whatever the choice, however, it should not involve you. Because if you get involved and/or make the choice for him he may never learn to make GOOD choices... on his own... which he needs to learn to do.
So, go ahead: Grieve. Scream. Rant. Cuss. Cry. Throw stuff. That part's for you, and it's okay. Tell your son you love him and how sorry you are that this happened to him. That part's for him, and it's okay, too. Then... tell him that he made a choice, supposedly as a man... and now has to decide how to handle that choice... as a man. Without you. That part... is also for him... and it's also okay.
I bid you the greatest of peace... and the strength to be a "man"...for your son. 'Cause he needs that... and NOT a "mommy"... from you, right now.
Your servant and a slave of Christ,