Exactly what others have told you... hand-reared, on bottle... he has no fear or respect for humans... regards you as one of his herd... and now he intends to assert his dominance... which may result in him killing or seriously injuring someone, as no human is a match for even an 900 pound Jersey bull.
Dairy bulls are famous for their aggressive, dangerous nature, and I don't know that they are inherently more dangerous than beef breeds - but the fact that they are removed from their dam at birth and reared in close proximity to humans is likely much more influential. Beef bulls, reared on their dams, in a pasture setting rarely display this sort of aggressive behavior.
We see similar dangerous (potentially lethal to humans) aggressive behavior from hand-reared stallions and llamas ( it is called 'berserk llama syndrome').
Opinions that he's just a 'rowdy teenager', and cautions to 'not trust' this bull fall far too short... he needs to be GONE ASAP, before he injures or kills someone!