He kissed his ex

This guy I’m dating kissed his ex yesterday when they were at a family outing with their kids . He said they’ve been broken up for almost two years. I found out because i seen a message pop up on his phone. We were supposed to be taking our relationship to the next level and be official. Should i let it go since we weren’t official? We made it official after he told me so now he can’t do that kind of stuff and not be held accountable. I’m just a little worried because what if he still loves her? Even though he said he doesn’t. He’s with me now so i need to try and let it go