try soft following gentle hands placed in the correct position which would be in line with his mouth, lots of bending, circles and trying to work him into the bit slowly. Use half halts and you may have to ask often at first, and using your seat and legs driving him into the bit. It could also be a bit issue, my tb mare was very unhappy with her regular snaffle and I went to a french link and it made a ton of difference. I would assume you also had teeth and mouth looked at and there is no back pain. These are the methods I used with my mare and she has really come along.