Should I end my relationship?

I have been dating my current boyfriend since september 2010 however we didn't become official bf/gf until february2011. Things have been moving really fast in our relationship and about three weeks ago we moved out of state together. Two days after the move I found out I was prego. I wanted to move back home because of this and although he wasnt happy about the move back he agreed. On the way back w stoped at a McDonalds I broke out the laptop while he was in the bathroom and decided to visit FB for a sec. The FB page was already open and it said there were 3 messeages so I clicked on what I thought were my messages and started to read them. To my surprise the messages didn't belong to me but to my bf. He had left his FB page open and I thought it was mine. The 1st message I read was from a girl asking why he had been ignoring her on FB and not txting her any more. She asked if he still wanted her. After reading through several messages she sent him I found out that the laptop he got from a "friend" a few months back was actually a gift from her. I also read that she knew about him moving out of state and she said if he ever decided to come back he would always have a place with her. I read another message that he sent some other random girl saying that she was amazing and beautiful and he asked her if he could get to know her. He sent that message about two weeks before we move. There is more but you get the point. I confronted him about it all and he says he doesnt want anyone else. He said that the one girl who gave him te laptop is a stalker and she wont leave him alone even though he has tried to end things with her. I don't believe that. He proposed to me a few days after I found this out and I said yes because I do love him but now about a week and a half later I find myself unable to let go of he hurt and anger and I question if I should be with him at all. He is 23 this month and I am 25 I think he still has some grow n up 2 do and I don't want to get hurt in the process. Should I give him another chance or once a lier and cheater always a lier and cheater? He claims he never cheated on me but pursuing another woman talk n bout get n ta know her is just as bad to me. So should I end my relationship?