baby daddy cheating?

I have a 4 month old and I live with his dad. We have had problems in the past with him cheating and today his phone was on the bed and I went to move it and it had an alert of a chatting app. 
This is not the first time I found him on a chatting app but I been too scared to confront him. I downloaded the app and it's chatting and video chatting and literally everyone on there was like I'm horny and showing their private parts. 
I need help I'm a wimp idk how to confront him. 
What would you do