Your h is having a "life" transition. Each and every one of us goes through them every 10 years or so. What triggered his may have been the birth of your second son. The reason that I am saying that is because he is now a father of two young children and is responsible for ensuring that his family is taken care of. Responsibility has been weighing heavily on his shoulders and many "young" men tend to have a problem w/starting or adding to families, especially when they are just really taking off on their careers.
Even though he's sober and been to rehab, just by his comments of being terrified and talking about his guilt, it puts me in mind that he may be at the very tail end of replay/depression. He's still got a ways to go and I wouldn't reconcile too quickly because he needs to show by actions, not words, that he's ready to come back home and be a husband, lover, companion and a father to his children. He still has some hard and necessary work to do. Drinking and drugs are escape tactics and he needs to face whatever issues he has before he can even begin to work on reconciling.
I would start out as friends and keep my expectations at zero. He does sound like he's finally seeing a little bit of day, but time will tell as to whether he stays the course or if he runs back up in the tunnel and seeks out his escape tactics once again. It takes a lot of work and determination to stay sober and I do hope he continues to heal, but he's got to face those demons from his past in order to move forward and grow up. You can be there as a friend, listen to what he has to say, but do not offer to fix his problems for him. This is his journey to help him grow up.
Keep the focus on you, your children and your finances. Find things that will keep you busy and yet, make you feel better as well.
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.
