He has a girlfriend, what part of that are you not getting??? He’s enjoying flirting and carrying on with you, because he considers it not to be cheating, even though it is. If you give into it, you will also be guilty of insinuating yourself into someone else’s relationship.
Try to think of it this way. If you had a boyfriend and you lived in another state (maybe you moved there because of school, maybe you met him on a trip and therefore you both live in different states, or maybe he had to take a job in another state to get a job, or a hundred other different reasons). Everything was going fine, until one day, you found out that your guy was chatting up a girl at work, then he pretended to make it sound like everything was completely platonic, even though you found flirty texts and phone messages with this other girl. How would that make you feel? Would you want to meet this girl and invite her into your social circle? Or would you want to set the record straight with her and inform her that “your boyfriend” is in a relationship? And, wouldn’t you want to go straight to this dude and ask him what the h*ll he thinks he is doing?
Just because he’s not dipping his wick doesn’t mean that he is innocent, and it doesn’t mean that he isn’t having an emotional affair, or that he isn’t leading on some other young women, in addition to you and his so-called girlfriend.
If you don’t mind being in a casual relationship with this guy, where you know full well that he has an official girlfriend, and you don’t mind hurting this other woman, then by all means, full steam ahead.
You already know that this dude is a player. Is that what you really want? If you confront him about it, he’ll try to tell you how misunderstood he is, especially because of how shy and confused he is. Isn’t that sweet? NOT.
If, and only if he breaks up with his real girlfriend, and he explains the whole situation to both you and to her, and about 6 months to a year passes and he hasn’t hooked up with anyone else, then I’d say it was safe to attempt to date him, but even then, I would have a list of deal-breakers to give to him. He’ll probably run so fast, he’ll leave skid marks.