I’m not sure what he means when he says that it’s just not there anymore.
Living I am speculating here but I am guessing he means he is not getting the same feeling from the marriage that he go when he had an affair. Affairs are secretive, "exciting" etc they release dopamine. The thing is that feeling doesn't last.
There was a poster on here who ended it with his W after a second affair. The last I read she was still single because dating for her wasn't as exciting as having an affair.
The problem is your H's head is too far up his a$$ to see that right now. I would venture to bet he truly believes you will always be there to catch him if this blows up in his face.