I would explain to your parents that he makes you happy, and that that is important. Why do they hate him? Tell your parents all the good things he does and why you like him. Maybe they just have the wrong impression of him. If they counter with the “nerdy” image, ask them why it is so important for him to be tall or an athlete.
I’m terribly sorry that that paragraph was incoherent. Best of luck to you both :)