You've done a great job in not reacting or questioning him. The best thing you can do is listen and sift through the garbage of what is coming out of his mouth to learn things about him and what he's up to.
From your posting, I am thinking that he is looking at you as a friend or an authority figure and that he's checking in to let you know what he's doing after work. In other words, just like a teenager letting the parents know that they are hanging w/friends and may not be home after the show. If he does it again, just say "thanks for letting me know". There's really not much more you can say because if you try to control and/or manipulate him into coming home instead of going w/his so called friend, he'll resent you for it and say you are making him give up his friends.
The affair has to die a natural death. A death by their own hands. The more they are together, the quicker the euphoria of newness will wear off. Your h is looking for affirmation/validation when he's telling you what he's doing for this person. Apparently as a child, he wasn't recognized for the things that he did, so in midlife, he searching for that from anyone that will give it to him. I suspect the ow is stroking his ego when he does things for her and that's what's got him going over there to do things for her...it will get old in due time.
In the meantime, if you have some projects that need his attention, you may gently ask him if he could help you out or better yet, ask him if he can tell you how to do them or suggest the help of someone else. It's time to start doing some 180's and this would be a good start. I don't expect him to jump up and say he'll do them for you, but at least you'll be doing something different. Whenever he does something for you, be sure to thank him.
So, pull up a chair and get comfortable, because the parade is about to start and you need to watch it zoom by. It takes a long time for these crisis people to come back to earth and the best thing you can do is set boundaries, adhere to them and get on w/your life the best way that you can. They are going to do whatever they have to do to get through their crisis.
Keep the focus on you.
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.