Should i trust him??

Ok well i am in a relationship with a guy who has a good heart and i think he loves me and i am def. in love with him. but the thing is we have been in a ton of fights about me not trusting him. but it is hard for me not to trust him cause ive seen and heard him flirting with other girls and i have also seen messages on facebook. the messages are to other females. i dont mind him talking to others girls but when he tells them he dont have a girlfriend and that he wants to hook up with them. then on weekends he goes out with his "friends" and he dont come back till the morning. his explantion is that he got drunk and passed out. i just dont know i am in way over my head please help me out.