Please help, I need advice

My fiancé and I have a wonderful newborn baby together. He's two months old and just precious. This morning when I got up with the baby I got on my partners phone and looked on his snapchat ( I did this because just last night he turned his phone out of my view very sneakily when I sat by him) and I noticed he had sent a snap to someone named Lydia. When I opened it, it was a naked picture of her to which he said "I can see your pussy :)" I lost it, gave him my ring and told him I don't trust him. He has a wandering eye (always checking women out in front of me) and he follows a bunch of "sexy girls" on snapchat. I'm currently just disgusted but he's begging me to give us a chance. What do I do???