How can I bring back the spark in my relationship?

In all of my past relationships, once the guy feels like the initial spark & feeling of a new, exciting relationship is over, they leave.
My current boyfriend just recently told me that he feels like that feeling is going away, and in his past he has never stayed with someone after that feeling goes away.
He is flirting with this one girl, he doesn’t know I know about, and it’s probably because he’s looking for a way to get that sparky feeling back.
I’ve sat down and talked to him about how he’s feeling, but nothing is going anywhere.
What can I(we) do to help bring that spark back, or at least get through this?