18M Getting belittled for my height.

My cousin loves belittling me for my height, its not even playful at this point. He's 3 years younger than me and has a really big ego or pride in being 6 feet. Every single time we meet he compares height with me and says how small I am, why are you this small? I didn't know you were this short and he does this especially during family functions or when other people are involved. Goes around other people comparing heights with me. The worst part is he is so serious when telling it like my life is over, saying it every chance he gets, when I say it's not midget level short he goes above and beyond to disagree with it and change my opinion, like he wants to convince me that I'm cooked forever. It's like he just wants to establish dominance over or wants to let me know that I'm somehow less value than him. I am also in a perfectly happy relationship and one time he made a comment on how did I even get a gf(assuming it had to do with my height), literally none of the elders, friends or even my gf (she's short) has a miniscule problem with my height, or no one really comments on it. I almost lost my cool today when he said "to be a man you must atleast be 5'10." How do I counter this? What can I say back? (im 5'6)