Bluesea, what I would remember is that this email is WORDS.
Believe nothing he says (or types), and half of what he does.
When my W started to move towards R and back towards the MR she said something similar.
"When I think about staying vs. going, it used to be (a few weeks before) that staying depressed me and going excited me. Now as time goes on I'm starting to feel excited by staying, and less so about going."
Now, remember, my W was not in an active affair. Her EA was over by this point (I saw her going through the grief period of the loss of that R). I think someone actively cheating saying something like this has to be taken with a huge grain of salt.
So if his feelings about potentially staying not being so dire to him any longer are genuine, then I believe it is a step in the right direction. However, I do wonder if it is a manipulation attempt. As in he has no where to go so is trying to buy more time as he detects you getting more fed up and finding the current situation untenable. What I would say is that for your decision, about whether you can continue on this way or not, ignore his words including this email. IN fact, that is what detachment is: moving on with what you want regardless of the WAS' words and actions.
Personally, I would ask him to leave. It sounds like an unhealthy situation with him coming and going to his other life whenever he feels like it, and the effect that is having on the rest of the household. I think of my father's horror stories about his alcoholic dad (I never knew my grandfather) coming and going all hours of the night and the effect it had on him and his siblings. I would not put up with unhealthy behaviors that are affecting the kids.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018