Is he cheating on me?

[Shiann]

My boyfriend and I have been together for 8 months now and recently I found him on a dating website called Meet Me. His profile pic is him, without a shirt, flexing. I scrolled down through his posts and he's been flirting with girls, asking them to hit him up, if they're down to do something tonight, just all this flirting. This led me to log on to his Facebook and read his messages. He's been flirting there too. Calling girls gorgeous, talking to his ex who he was with a few months before me, and referring to me as "some girl" or "Nothing important". He won't change his relationship status, won't tag me, or even let me comment on his pictures. It's like I don't even exist. He always tells me that he's going to go to the gym at 12 or 1 am and every time I text or call him he won't answer for hours and always says he left it in his locker. Do you think he might be cheating?
He always accuses me of cheating, too. And sometimes, late at night, he says that he's out taking a walk or at the gym and this is at 12 or 1am. He lives in Norwalk, one of the sketchiest cities in L.A. and always says that he had his phone in his locker or it's dead. I've caught him flirting before with his exes and even some girl he met off the Xbox. When I confront him he gets all lovey dovey with me and tries to kiss my ass.