Ambivalent, Please stay positive. Don't "assume" that what he's writing will be horrible. He could very well be writing to tell you what is bothering him and why he needs to be on his own for a while.
Mlc is not just about going back to see what they missed, i.e., parities, etc. It's going back to a place in their early years where they were emotionally stunted, i.e., most likely in your h's case when his parents' marriage began to fall apart. If that is the case, that is when he wasn't validated and/or received affirmation from one of his parents...most likely the father since he was in mlc. It could be that because he was considered the black sheep he wasn't validated and they didn't take him seriously. He may have felt abandonment when his father left and had a front row seat to the mlc. As a kid, he may have had to take on more responsibility than he was ready for and his coping skills didn't reach maturity, most likely he had to fake it to make it while growing up. In other words, he grew up too fast and the emotional well being didn't catch up.
He will resolve his issues one way or the other. They don't necessarily need to speak to the parents, but the review of old photos, where they lived as children, speaking to relatives and yes, eventually just sitting down and talking to a counselor will help. In some instances, he may even come to you and talk about it. It all depends upon how "safe" he feels talking to you.
Ambivalent, from your postings, your h has been one of nicer ones and that is always a positive. Had he been one of the nutty buddies, he would have been long gone, nasty and done some terrible things. I do think that your h is trying to work through his issues and he's going about it the safe way by being w/others, bowling, games, etc. I could be very wrong, but I don't think your h wants the marriage to end. I think he's one of those who just needs some time by himself to figure things out.
I know you are very hurt and it's going to take some time to settle down, but when you do, things may be a bit clearer for you. If he does send you an email today, read it and then read it again. Do not respond back in a knee jerk way. If there is something in the email that doesn't sit well w/you, sit on it for a while and then respond. We, as humans, sometimes tend to respond back in a knee jerk fashion and that's when we say and do things that we later regret and we can't take back.
Please take care of yourself. Come here to vent or just to post. We do know what you are going through and it takes time to work through that anger and hurt.
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.