How can i trust him after this?

[Robes]

First of all, how did you get on his facebook? I don't let anyone into my fb/phone so your invading his privacy there. Anyway, us guys are always thinking about other women even if we have a girlfriend. Same for girls with boyfriends fantasizing about other men. But I'm not sure what your describing here. Was he actually talking to other girls through messages? Or was he talking to his friends about other girls? Two different things. If he was already talking to other girls then he probably wants to get with them. If it was the latter then MAYBE he hasn't done anything wrong. Again, not really sure but talk to him and figure out what's going on. If you still don't trust him then leave him.