Three some (long)

So he left his work phone at home when he went to work and it kept going off so I went on it thinking it may be his boss. I got curious and looked through his messages. He was texting a girl saying how bad he wanted to “fuck the shit” out of her. She said she wanted a 3 some with one of my mans friends and he was “so down with it”. He texted her multiple times asking when they can do it. I can’t tell if it ever happened or not. I just texted her from his phone and am waiting for her to text back. I need to know if it happened or not. We have a three month old son together and I’m DISGUSTED. Never thought our 5 year relationship would go to shit like this. What should I do?!?! Please don’t just say “leave him”. I know that leaving him is an option and am thinking it through. I honestly don’t want to breakup with him bc I really love him. I know I deserve better than this but I wanted to be with him forever 😢 please help me.