Oh, if bolting is your problem then no bit in the world can fix it! If a horse wants to bolt, they WILL bolt. A harsher bit will only cause more discomfort in those moments of fear and will making him want to run even more. I totally agree with Jubilee Rose. Start at the basics. Get out of the saddle, take the bit out of his mouth, and do some good old fashion ground training. Get a trainer to help you. You need to build a strong foundation with you horse on the ground before you ever try to fix things in the saddle. I harsher bit will only make things worse. Harsher bits are meant to refine a horses already learned skills, not to try to hammer the skills into a green horse. When you do return to working with a bit, get a snaffle that fits him properly, but definitely start with a snaffle.
Jubilee... the other Jubilee ;)