With a wood block and some rawhide cream, bosals can be shaped to fit any horse. And if you don't think a bosal would work, get a snaffle with bigger rings. The heavier the bit, the more response. And if you get a large ring, weighted snaffle, he will have a hard time sliding it through his mouth. Chicks saddlery has some inexpensive ones that work great.
~Started young horses in Bosca te Ador, unto the two rein the old Spanish spade, brought them along with two hands that were gentle. Some fine reining horses as ever were made~