I am truly sorry that you are here...but the entire forum has some great folks posting and providing advice.
First, they tend to have triggers 18-24 months prior to the BD. Evidently he knew his friends were leaving their wives, but did something else happen like a death of a family member or close friend, the loss of a job, a raise w/more expectations from the boss, the birth of a child or even a health issue w/him or you? Once the triggers have their switch flipped, there is no going back for them.
Yes, MLC is all about the trauma from their childhood. They were emotionally stunted at an early age and now have to face their demons from the past and figure out that they were not responsible for what happened. In his eyes, you represent an "authority figure". So, he will lash out at you because you. You do not want to come off as a mother to him.
Second, the stages of MLC are just a guide. The timelines are just a guide because each and every person that goes through a MLC will go through it at their pace and it will take as long as it takes from them to heal. The stages are not linear and they can bounce back and forth in them, i.e., just as a person who is grieving.
For now, focus on you, dig deeper for patience and detach as much as you can. Try to look at him as a roommate and yes, they do become the mirror image of the person you love (exact opposite). Keep your expectations at zero and all times.
Hang in there! You've got this...it just takes time to find your footing.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.