You're 27yrs old and single. You seem like you have an excellent set up with your ex - all those free nights should be having fun - not squirming over some guy who may or may not fancy someone else.
You don't live together, you don't have kids together, you aren't married, in the grand scheme of things he's been in your life a blink. He doesn't really owe you anything and you don't really owe him anything.
All that is at stake is hurt feelings. Better to know now though right? Before it gets any deeper before commitments are made and kids are introduced.
And with that - well done for the no introduction to kids rule! That was really sensible because you now see first hand why that's so important. You sound like you have your head screwed on.
If I was you, I'd just back off from him. Start spending time with some of your friends. Get out a bit. Try a new hobby. Join a group etc etc.
If he does go for the other girl, there isn't anything you can do and he's not even really an arsehole for it, it's what happens in your 20s when you're looking for someone to share your life with - unless he starts to string you along - then you can't really be too angry with a guy in a low commitment relationship having his head turned. It sucks but it's also just life. I'd just start distancing a bit now - he's showing you those red flags that his head is turning. My only words of wisdom are: Brace for impact
You're seeing the signs, there's a tonne of stuff you can do to lessen the blow if he decides he's sorry but he does want pursue things with this other girl.
Like i say - it sucks and its 100% ok to be upset. It will hurt a while but like with any relationship in this scenario, you will move on.
May I recommend Amy Winehouse's - Tears dry on their own song. The line 'i dont understand why i stress then man, when there are bigger things at hand' and 'I should be my own best friend not eff myself in the head with stupid men'. It was my soundtrack of my 20s break-ups and the lyrics sprung to mind when i read your post.
You'll be fine - you're so young. I so wish, so so wish - I could go back to mid-20s me and slap her in the face for the amount of time I spent hung up on heartache. The things I could have and would have done if only I'd realised.