I think a bull would be analogous to a stallion, not as tractable as a gelding (steer) or a mare (cow). Adult male animals can be problematic. We had a ram who was raised as a pet and would never stop ramming us, out of affection. We had to put him down. Alas, poor Harold!
Please don't. Get a steer, or if your getting a bull to breed some cows, afterward steer him, give it some time then procede with great caution. Bulls are dangerous. And tame bulls are more dangerous. They can be gentle and easily managed. Until its time to breed. Then they only have one thing on their mind. They can smell and/or hear a cow in heat from many miles around. Bulls should never be made friendly. They don't give one shit about their training, care giver, being respectful, even fences unless it's seriously bull proof..... they are big, strong, an potentially dangerous..... Get a steer. They are the ox. Don't mess with bull testosterone. Them giant things hanging down there produce a lot.
Better yet is take one of his bull calves, steer it young an raise to be you riding critter... Much safer, easier.
I've seen it, but they have all been bull calves that were raised, well, basically in the house! As mentioned above, "project" calves, pretty much attached to the person who rides them with trust established since birth.
I've trained horses to saddle (hate that term "broke") and I suspect a bull would be no different, there are some that will be easier and some that will be harder, maybe the fact that you're even thinking about it means that this bull is not a wild beast.
Hopefully bull already has the ring in its nose, as putting it in there probably shouldn't happen anytime near training so they don't associate riding with pain.
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