Do they know an approximate age on this horse?
Depending on how old he is, he may be so aggressive simply because he's a stallion, and that's just the way he is. Gelding him (if he's older already) may not even improve his behavior much.
Also, if he is more 'aggressive' by nature, and he was abused because of it, you have a LONG LONG road ahead of you, because all he's learned out of life is that he "has" to defend himself. All I can say is be very careful, and never step lightly around this guy, even with Patrick there.