stay cool, flipping out is a sure way to let people know. just confront him in a reasonable tone. if he acts like a bitch, just look him right in the eye and tell him you could talk about it more outside. once out there, treat him like a bitch.
The best thing to do is...like others have said...go tell mgmt.
However, if it was me...i would go up to the little fucker AFTER hours, or off gym property...and get all up in his face...most likely he will back down and say it wont happen again...however, if he talks smack, i would rip him a new one!!
dont worry about it bro, take it as a compliment. there are gonna be tons more guys just like this little shit head. and u cant go kick all their asses. just forget about the little runt, he will quit soon anyway, they always do. the bigger u get the more dick heads there are, just remember when he is gone there is always someone else that is gonna piss u off.