I am very confused.
You first got a letter saying he was going to be charged??? This doesn't happen. To be charged there has to be enough evidence for a successful prosecution. Are you sure your b/f wasn't arrested first, or maybe refused what is known as a contemporaneous interview???
Are you also sure they stated it "not in the public interest"?? All violence is in the public interest I am led to believe. May be "Not enough evidence to prosecute"???
I think you are getting "charged" and "prosecuted" mixed up. You can not go to court without being charged in the first place.
If however you are correct with your terms and facts, then he will clearly get off with a technicality.
And finally if he admitted (assuming to you) that he grabbed the other mans collar, why didn't he just admit it to the police and receieve a caution?? I imagine, therefore, he has some previous.
Sorry, there are too many holes in your thread to give any structured advice.
But wish him luck as a Commando. Good career choice.