Unloyalty

Okay, so me and my partner have been together for 8 months now, he's 21 I'm 18. I did some snooping the other day, which I shouldn't have, but I logged onto his snapchat and found that he'd been asking girls to hang out etc etc, not to mention this was 11 at night while I was at work, this has already happened before, October last year, I don't know what to think of it all?!? I need some serious advice, my best friend is telling me to leave him, I can't imagine my life without him but doing stuff like that isn't okay.. When you love someone being faithful isn't hard