M.R.
This jumped out at me though it was a long way into your post: "His father always used to boast how proud he was off my son for having such a great wife and a happy family etc."
Please stop and think hard about that. Did you too do the same -- take pride in your son's marriage, his perfect relationship, etc.? It is possible that your son did not tell you about his divorce until it was over because he knew that you and his late dad placed a VERY high value on his being perfect -- "this happy, fun loving, forgiving and peaceful guy" as you put it, who had a perfect job, perfect home and perfect wife. He probably felt that he was a letdown to everyone; after all, wives don't cheat on perfect guys. Perfect husbands don't get divorced. Perfect sons don't confess to mom that their wives cheated with their old friends. So of course he is clamming up now and distancing himself and working to fill the void.
You seem very puzzled that he would act like he was never married, refuse to acknowledge the existence of his marriage or his ex-wife, and stick all his marriage memoribilia into a box (which by the way wasn't yours to open). But can you see that right now, so short a time after being made a fool of, he needs to isolate that whole part of his existence so he can just get by day to day? Let him. Don't press him to talk.
I suspect you are just now realizing your son is not the ideal man you thought -- especially since he would get his wife's lover fired, which is, I agree, very low and wrong of him -- but it's human. You'd prefer that he handle this divorce like a gentleman, no punches, no firing, no sticking his past into a box. But he isn't handling it that way, and so you feel you no longer know him: "It just isn't really my son at all...it's not my son to do that...." You are seeing your son's humanity, anger, pettiness and vengefulness come out and you are surprised by them -- which is totally understandable since he's never had reason before to act out like this. But it won't help him to have any inkling that you are disappointed in him. He surely already assumes you are disappointed since he knew for years that you and dad placed such a value on his ideal marriage. Now you are also seeing his less than ideal reaction, and you are disappointed in him. You have reason to be. But there is nothing you can do to undo the firing or the punch or the anger he feels.
Please don't ever mention his ex-wife to him; if you are in touch with her yourself, stop being in touch --there are no grandkids to link you to her, and she cheated. (How do you know about his getting the guy fired? Did your son tell you? I find it hard to believe he'd tell you that--which makes me wonder if maybe you are in contact with his ex?) Do not tell him you went into that box; say nothing. Don't criticize his working hours. Certainly don't mention his so-called friend, ever. Don't let slip that "You and Sarah seemed so right together, I just don't see how this could happen."
Find ways to spend time with him and do things where his attention can be on something else-- a show, a play, a concert, a festival or fair, anything except sitting around and talking. He needs distraction. For yourself, this is a tough time as you realize that your ideal child is not ideal, but once you get past being puzzled and hurt by that, you can shift your focus to keeping in touch with him and doing things that allow him to be with you without having to talk much for now.