What should you do/what does it mean?

[Èxálté01]

It means he can't be trusted and he doesn't think about how that would affect you negatively. If he flirts online, there's a higher possibility that he also flirts with other girls in real life. His age though is one factor. He's still immature. But that doesn't excuse him from flirting when he has a girlfriend. Tell him to stop and explain why. Ask him how he'd feel if you were flirting with guys online. If he still doesn't get it then he can get lost.