Just found out that my guy has slept with 3 other girls in the past 2 weeks...?

Okay, so I've been seeing a guy for almost a month, and I just found out that he had a with 2 girls on Saturday night when he was supposed to spend time with me, and last Monday when I was sick he went out with his friends and lied about it and slept with some tramp at a club. I'm devistated... and he's being a total as$ about it. Can you please give me some advice?