He is back with her. Since she was an “ex” there is probably a reason. She likely will be an ex again, as soon as the problem, whatever that is, poses itself again.
He likes leaving his options open. You are merely one of his many options. Don’t allow yourself to get sucked into that kind of situation.
I’m sure he would say things to you that sound tempting, but just vague enough so that he has an exit plan, for when you, or the other woman become “inconvenient” for him.
He will probably say something like, “I never said that!” or “I’m sorry if you got the wrong idea” or “I was honest with you.” So he doesn’t have to look like the bad guy.
Move on and find someone who you really like, that really likes you too. Someone who is 100% available and looking for a real relationship.