How to cope with bf who online dated

Together for 6 years now have a 5 month old son. He never made me second guess him. I gave him my trust 100% never doubted him because we’re best friends that tell each other everything and he always takes care of me. Until recently I accidentally went through his saved account info on websites through my iPhone I didn’t know was in my phone. We’re on a family plan. That’s when i seen 7 different dating websites with his username and password saved so I was able to get into. I did but found no messages. I did some more digging because I was becoming suspicious. Found messages from two girls saying let’s meet up. Him responding back saying how sexy they are and calling them babe sending each other nude pics. Him saying his sex life is boring. Also him asking the girl you think we will be in a relationship and move in together.

Btw these messages was 3 years ago.

I confronted him about it he kept saying he didn’t cheat it was just turning him on. He did it to get off. I don’t want to throw away 6 years of our relationship and we have a son now. How can I cope with this? He wants to take me out to a really expensive dinner next to the ocean tomorrow just us two but I’m still feeling really hurt. I feel insecure now even though that was three years ago but I feel like he already cheated on me.

I want to fight for us but I keep thinking about those messages. I feel like having sex with him is going to be hard too.