How should I approach my partner

I have had a gut instinct that something wasn't right with my s/o and put a post up, I couldn't help my self I ended up looking through his Facebook and found some stuff that is not proof he has been lying but I'm definatly not ok with- he has deleted parts of conversations and his search history is full of other girls, 2 ex's my best mate and other girls I don't know, the same girls over and over the last month, he has "waved" at one of the ex's who isn't a friend on Facebook and I know they didn't end badly. and 2 other girls I don't know. (to me seems like trying to start a conversation) bit of back story we had been fine and happy but things started to not seem right with him and he got funny with his phone all in the last month, this ain't the first time he has lied about some messages I had seen, he got a message from a girl and lied about what it was. they had been speaking before we got together and that's fine but he lied and just said she was a work friend who he was trying to get rid of. we spoke about it and all was fine. I don't think he would physically cheat but I don't trust that he wouldn't message other girls. how do i approach him without seeming like a crazy bitch