Most Helpful Girl
Well, for some people flirting with other people is cheating, so that depends on what you consider flirting. If you're OK with his behavior , then that's fine, but we don't know if he actually meets up with those girls. Problem is, you can't really confront him with his behavior , right? So what you could do is make up a story. Like: my friend X told me that her boyfriend is flirting with other girls, like meeting up behind her back with other girls, telling them he's naughty or dirty or something, ... See how he reacts.
Sorry if this is a bit lame, I don't really know what else you could do.