What do I do..

I've found out my partner is using snapchat to chat with a couple of girls. They're flirty texts. I can see them because I got suspicious of him getting messages and kinda facing the phone away from me. So I looked at the snapchat on his phone and he has it set so some messages dont disappear for 24hrs.

I dont know how to approach this. As I don't want to be like yeah I looked through your phone. I really hate drama confrontation and arguments.

We've been together almost 2yrs, live together and have a 4month old.