Although I have no cattle to move, I actually have desensitized my mare and a couple of other horses to swinging whips, including bullwhips, from on their backs. I know it's not the only way, and there might be a better way, but this worked well for me.
If you've got him so that you can crack it around him, great. What I would do is make sure he knows what he's hearing -- walk up to him, let him eye it, sniff it, whatever he likes. Rub him with it on his neck and sides and just let him know it's not anything to fear. Then, with him in a halter and longer lead, (or loose if you think he's fine that way,) swing it around him; let it land lightly on his shoulders and around his feet, just all around him. When he's comfortable with that, hit it on the ground and make softer noises with it. If he's taking that well, rub him down again and make sure he's really ready before you decide to really crack it. If you can crack it from somewhere near him without him flinching hard, he's ready to move on.
Transitioning to riding was easy for my mare and me. I got on her and I swung the whip around her like I had the days before, very carefully and gently, and just let her move around while I did so. She was relaxed and calm, so I popped it lightly again and then went back to swinging it around her. Well, after a while, it was as though the whip wasn't even there, and after working my way to louder noises with it, I can now crack a bullwhip from off her back as well as a lunge whip.
This was all over the course of two or three days a few summers ago, and shortly thereafter, I was able to ride her bareback while swinging whips around and cracking them. She really is coming close to being bombproof, (if there is such a thing,) and I think desensitizing them to things like this can really make a difference in everyday riding, whether on the trails, in the pasture, or in the arena.
Candy | 14.3hh sorrel AQHA mare
Diesel |15hh bay grade gaited gelding