NTA, I don’t know if he’s deliberately trying to flirt with your girlfriend, but even if he is as long as you trust her it will be fine. The more upset you get the worse it will make you look to your girlfriend. However he does seem like a tool and quite the asshole. The smart play would be to just to be nice even if you have to bite your tongue as it will be a lot more effective. Revving your engine even if you have a cool exhaust will make you look worse

Just play it cool, be overly nice and don’t let your girlfriend think you’re insecure or jealous. If it gets to a point where you think she’s doing anything, just call it out, but don’t start getting upset prematurely.

That’s the best advice I can give as my ex gf worked in a restaurant and was flirted with constantly by all the male servers and kitchen staff. They don’t see it the same as we do unfortunately they think it’s harmless and they’re just nice.