Put yourself in his shoes, would you want some PhD student making your job awkward. He may even be spoken for. On the other hand you may not be as hot as you think you are, and just flirt due to a low self-esteem, in which case get help. You will gain nothing and lose all by harmless flirting. On the other hand if you want to get with him, then flirt away, and since you seem to flirt freely, why not aim to have a research group orgy?