Your going to want to start out riding your horse with two hands, using a fair amount more contact then you would on a finished barrel horse. Get a gentle bit that the horse goes nicely in and accepts.
I would highly suggest starting out with a snaffle for barrels. When barrel racing, you want to start out with an easy bit. Some horses grow out of the snaffle when they get more seasoned to the speed events. But for now I would for now I would go with the snaffle. I have a year old that I just got from Tennessee and she is in a snaffle and is doing pretty good with it.
I like Hackamores for starting a horse out in barrels. But your horse may not respond to it as well as others do. I suggest trying both a snaffle and a Hackamore:) See which one he responds to better:)
Since no one has mentioned it yet, if you go with a snaffle, I would suggest a D-ring or a full cheek to keep the bit from being pulled through his mouth as O-rings often do. Your only other option with an O-ring is to keep a bit hobble pretty tight but that would confuse the signals.
Morganshow11 - I will start him on the bit, and if he doesn't respond well to that I will try a hackamore. My uncle suggested a Novel Headstall, but I have never seen anyone use one of those while competing.