I would use a fulmer (they are hard to find in the states now) which is a full cheek with a loose ring- this bit provides stability in the horse's mouth, the full cheeks fixed to the cheek pieces with keepers keeps the joint of the mouth piece in correct alignment no matter where he settles his head as he is figuring things out, and the loose ring forgives any movement in the rein from the rider's hand while everyone is finding their way. I would then go back to the basics. Make sure he understands the concept of going to the rein contact, accepting the bit and moves into the reins evenly- meaning steps off evenly with both hind legs. Uneven contact in the bit generally comes from two sources-either uneven rider contact (unequal in the left and right rein) or a crooked horse. The way to correct a crooked horse who is healthy and without physcal limitations which create the uneveness is to go back to work in hand and on the lunge to move his crooked parts back into alignment so he is straight from tail to nose. Refer to Podhajsky's book-The Complete Training of Horse and Rider. He was the director of the Spanish Riding School and explaines contact and straightness in detail.