Personally I would keep him in the snaffle unless he absolutely needs a bit change. I started my colt(now gelding) in an offset dee, and he absolutely loves it and responds very well to it. I want to keep him as soft mouthed as possible, so I'm keeping him in it until he needs a change. He is four, has been under saddle for almost two years, and starts his real barrel training this year. My mare, however, runs in a Tom Thumb, which isn't necessarily a good bit but for her it works. She blows right through a snaffle.