TWH (even flat shod ones who have never been in stacks or sored, etc.) trip a lot. The barn I'm at shows walkers (again, flat shod) and they often have tripping issues. It comes from not being balanced at the gait, I believe. Gaited horses need to be balanced by the rider to gait correctly, and when the rider isn't balanced, the horse isn't, and vice versa, so then they start tripping. Hope you get it sorted out, and that may give a little insight if his feet are fine & such.