I ride my mare bitless and my gelding in a Robarts pinchless D ring snaffle. There are no "must use for gaited" bits, even if people try to sell them to you.
I personally prefer Mylar and Robarts bits. When I do put a bit in my mare's mouth its the Mylar short shanked comfort snaffle (has the roller center).
But you just have to see what the horse likes, it all depends on the shape of his mouth. Training comes first, and if someone has to depend on a harsh mouthpiece to keep control, they are skipping out on training.. so technically the bit should not matter other than comfort considerations, and knowing that long shanks in uneducated hands can do a LOT of damage (due to higher leverage).
Try him in a standard snaffle, if he's tossing his head around he may not like something poking him in the roof of the mouth, try a french link or something with a roller center or mild/low port. Always start with groundwork and establish yourself as the boss, its just easier that way.