The guy I like has a girlfriend

I’ll try to keep this as short as possible so there’s this guy who I’ve known for a long time. We recently reconnected and went on a few dates that were so much fun. Unfortunately one night we went out we got blackout drunk though and I ended up finding a condom wrapper the next morning. Although neither of us remembered what happened that night.

We continued going out together and going on dates then recently I realized how distant he’s been. He went from calling me 7-8 times throughout the day to me barely getting a text back. He’ll just text me at the end of the day and tell me his day was shit and he’ll talk to me tomorrow.

So I did digging & I seen him and a girl kissing on his story on the same day he ghosted me. But that same night he was texting me saying he missed me ??? I’m just hurt and confused because I really liked him and to see him literally in a whole relationship just makes my heart sink. Like everything was a lie…

He doesn’t know that I know, what should I say to him? Should I say something to his girlfriend?