His issues with relationship do stem from his relationship with his mother and if he wants a normal happy life he is going to have to take counseling seriously and stick with it. He also exhibits some characteristics of a Midlife crisis because he no longer knows what he wants out of life long term this is a pretty sure sign that somewhere along the line he has lost connection with who he is. He lost sight of what is important to him and what really matters in his life and that could be in part due to his mother and his past. He may well feel he is in no man's land and realize there is definitely no going back but there is no clear way forward towards the future. He seems frustrated with just about every aspect of his life and doesn't know what he wants for his life and the people that are in it. Dwelling on what can't be undone keeps him locked into the past rather than focusing on the future. Learning from painful regrets and using them as a example for change will help him move on and put the past where it belongs "in the past."
He needs to remind himself of what he has now and be open to all possibilities and be ready for the next new chapter in his life instead he is uncertain as to what he wants right now and that is due to the fact that he didn't have that bonding and nurturing growing up that makes most people productive in their relationships and able to bond with their mate. Midlife crisis can make him feel trapped like a caged bird and wanting to get out at all cost, even if it means losing the one he loves. Somewhere along the way, various dreams and goals may be realized and now he feels hemmed in by thoughts of what life 'could have been'. What's the first thing we do when we feel trapped? We look for ways to escape! He needs to realize that what he really wants is not to escape but for things to change. This is why counseling is so important for him because right now he is feeling the affects of the double whammy midlife crisis and past treatment and failures of his mother.
In order to get the help he needs he has to go into counseling and stick with it or else he may find himself alone with no one to turn to. The fact that he leans more towards younger women, shows signs of midlife crisis but could also mean that older women or women his age reminds him of his mother and her failures as a parent and then it scares him because he doesn't want to be left alone again or deserted emotionally. It's important for you to talk him into going back to counseling and sticking with it for at least six months or so he needs extensive counseling to help realize that his mother actions were not his fault and he deserves to be happy and loved and cared for and until he can realize these things he will always live in limbo not knowing what he wants or what he deserves out of life.