You do nothing. Back off again, and leave him be, and live your life as if he's NOT coming back to you, and like you're NOT going to be his "2nd choice."
It's good that you confronted him, and VERY good that you didn't tell him how you knew. Don't ever tell him that. Document the spending (a family court judge isn't going to look kindly on his spending family assets on a "love nest" for him and his girlfriend), and go about your day.
Easier said than done, emotionally, I realize, but that IS my advice to you. He will contact YOU very soon -- probably in a panic (unless he's even more delusional than I think he is). Unless it's about your sons, just ignore him.
