If you've handled studs before and are experienced in doing so you must then realize that they often will act up and rear. He's acting aggressively towards you because he knows he's the boss and he can get away with it. When dealing with a stallion you can't always get away with being gentle. Horses are big tough animals and while I don't condone any excessive abuse, sometimes you need to put a stud in it's place. Use a chain, carry a whip, use a "rearing bit", whatever it takes to get him to respect your space and respect you. As I've said, if you don't already know and realize this then you shouldn't be handling a stud and you've already let him walk all over you.