No need to be mean to the animal, but whack him with a bucket or something when he crowds you.

Animals that don't trust humans + humans that don't understand animal behavior = a recipe for disaster. 

In my way of thinking, if humans 'whacked' a dog everytime he got close to them, sooner or later that dog would bite someone.  I would apply the same thinking to the bull -- if humans 'whacked' him everytime he gets close to a human, and he is treated this way his entire life, at the very least you're going to teach him to never trust a human.  Add to this scenario, any human who doesn't understand basic animal behavior and the small human doesn't stand a chance unless he kills or severly injures the animal.  

Maybe try avoiding the situations where the bull likes to get up close and personal.  As a human, don't just stand still and let him walk up to you, move around the other animals, the feeder, fence, pickup, etc.  Don't have to run, just move around -- the bull will get retrained without associating the 'whack' with human interaction.

I'd have to agree with Galaxie64 below.  I don't want bulls around who fear or don't trust me, as a human.  I don't mind looking at calm bulls that you can get close enough to to sprinkle some fly dust down their back out of a shaker can.  I want all the animals to be able to 'trust' me and I strive for my actions anytime I'm around them to not be of the type that instills fear or anger in them. 

I do understand there are 'mean' animals 'out there'.  Those should be easy to identify as their actions are always predictably mean towards humans.  Those animals should not be a part of any herd.   There is a reason they act that way, they were taught not to trust humans either by their mother or by humans.