Well first of all, who says you can't change your mind? If he's making you feel uncomfortable in any way you certainly have the right to mention it. I'd even go so far as to say you have the right to demand that he stifle it. Personally (and you might see it differently) I would take it as a sign of disrespect, especially if he's doing it blatantly in front of you. If you make it clear it bugs you that much there should be no reason why he doesn't just chill out.
Not all guys flirt excessively, it really depends on the person and how social they are. I think men and women are equal in this (except men might be slower to take a hint).
I think flirting "too much" is when it starts to make people feel awkward, uncomfortable, or even annoyed, whether the person is the recipient or your significant other. Best of luck to you.
I have seen the mystics play there,
Once or twice but I knew they had a reason.
Enchantment plays it's cards all right,
Hand in hand with the working of the seasons.