Yeah, I'm gonna go out on a limb here and suggest that in all likelihood, his girlfriend doesn't even know you exist in a romantic context.

Don't get me wrong, there are open relationships out there of a "Don't Ask, Don't Tell" type - but those are restricted to casual sex and flings for the most part. I don't know any examples of true poly relationships - that is, multiple romantic relationships with multiple partners - that are DADT.

There is an easy way to find out, of course - ask him if you can speak to his girlfriend about the whole thing. See how she feels about it.

His reaction to that will tell you all you need to know - if he's like "yeah, sure, here's her number" etc, then he's likely genuine.

If he comes up with any excuse at all for you not having contact with her - he's a cheater, or at the very least, being dishonest with one or both of you.