PSA: He's really that into you.
Here is one of those simple facts of life women never seem to quite be able to grasp so maybe it must be said out loud: If a single guy approaches you at a bar and is buying you drinks and you are telling him over and over that you have a boyfriend but he says that's cool and continues to stick around is is not because he is hearing your words and respecting your relationship, it is only because your boyfriend is not there ergo he believes he stands a chance at getting some action. And sometimes that works. But if your boyfriend shows up and the guy who was buying you drinks suddenly makes himself scarce I can't believe one wouldn't put two and two together.
Then again it's probably because, and I hope to not overgeneralize, while women can be wicked men trend towards simply being scum.