wow your treading in dangerous waters, I mean more than about her you shoudl be worried about him. He dumped you five years ago, now he's flirting with you even though he has a girlfriend...maybe he isn't the kinda guy u wanna be with unless u want to get dumped again....
The obvious thing for him to do is to just end it with the gf ..but u might really want to consider if you want to be with this guy in the first place..unless u don't want anything serious
And since you know you cannot see yourself,
so well as by reflection, I, your glass,
will modestly discover to yourself,
that of yourself which you yet know not of.