How far along in training are you with this horse?
Just wondering because I think it works best if you introduce the bit LAST in the horses training. It is a difficult concept for them and they can easily be frustrated, as I'm sure you know, lol!
Also, have you considered starting this horse in a hackamore or bosal? Sometimes that can further the training without having to introduce the bit, it will also teach the horse that the bit is a form of COMMUNICATION and NOT control. You know?
When finally introducing a bit though you should always start with the least severe bit possible. Nothing with shanks longer than 4-5 inches, and preferably a simple snaffle.