I 23F found photos of women on my bf 23M phone and I’m not sure how to proceed

Hello, I would like some advice. I (23F) and my boyfriend (23M)have been dating for almost a year and a half. We have talked about what we considered cheating and having phone access to each others phone, but we have never really went through each others phone. The other day I was taking some photos on his phone and while I was going to delete some, I found photos of women from Facebook dating, they were screenshots of their dating profile. My heart dropped when I found out and I’m really not proud to admit it, but those photos led me to start looking through his phone. I ended up finding out he has a second account, and has followed multiple OF women and has just messaged “Hi” with the other recepients not responding. There was one that was sent last week where he messaged his girl about meeting up and when she had messaged that she has a boyfriend, he responded “what’s stopping us I have a gf LOL” and it made me so nauseous. I’m not sure how to proceed with this information, I don’t know how to confront him because he seems perfectly fine and there hasn’t been a change in his behavior. I have found out he’s been doing this beginning last month and we haven’t had any problems within our relationship. I just need advice on what to do?