If the bit has no shanks, it's a snaffle. If the bit has shanks,it's a curb--probably a Tom Thumb. Is this the bit that he came with? Are you direct reining or neck reining? As a general rule, these days most people prefer either a solid mouthpiece (low port curb or Kimberwicke, mullen mouth) or a double jointed (as opposed to the single joint/V shaped mouth) French or comfort snaffle.
The one rule to keep in mind: a direct rein bit is a snaffle and any bit with shanks or leverage is a curb regardless of whether the mouthpiece is jointed or solid. Leverage/no leverage determines the kind of bit, not the mouthpiece.
I like Myler bits, you can read up on them here: http://www.mylerbitsusa.com/
Even if you don't buy a Myler, there's a lot of good info there about how the different kinds of bits work.
If you google around a little on the internet, you can find a place that will rent you a Myler bit to use for a short time, then you can decide if that's the one that you want to buy.