Firstly, if he's been effectively taught to buck when hit with a crop, don't hit him with a crop! If you absolutely feel the need for a crop, then I'd start on the ground, teaching him not to fear it and how to *respond* to it correctly rather than react. Next I'd get him checked out physically to make sure it's not pain from his back, saddle, teeth, neck, feet, etc.
Then I'd reconsider your attitude. I don't believe for a minute that just because he's always done it means he can't be changed, but it does mean that he's had lots of practice & confirmation of this behaviour working for him in the past, so it will take skill, persistance, consistancy & time to change.
If you really believe he can't change, then I don't get why you're discussing punishing him for it anyway. But if he only bucks when hit with a crop, then it's likely this behaviour is(or at least was originally) a fear reaction anyway, so further punishment isn't likely to calm and reassure him. Whatever consequences you're going to use, be it reinforcement or punishment, it needs to happen instantly, at the time of the behaviour you want to influence.