If anyone is in that field, be it a friend, a relative, a hired hand, a ditchwalker, a vet, whomever, and they are injured by that bull when he plays with them, it will become a claim against the landowners insurance. Even if it's a friend, if they are injured and require medical care, which could certainly happen, their insurance company will sue the owner of the bull's insurance company. The owner will lose the suit because the bull is known for this behavior. It's your issue to deal with as you see fit. My concern was to make you aware that petting the bull on his head encourages a dangerous behavior. You seem to be aware of that.