Funny enough I think, after about $800 in bits later and 1.5 years, I don m figured out that he doesn't like lose rings! Such a funny thing, finally someone I randomly meet suggested it and it all fit together. He's gone best in cheap full cheeks, but I don't like using bits that aren't top-of-the-line on my baby, and he fidgets the most on loose or no contact. I ordered a Blue Bombers anatomic, sweet metal full cheek with a roller and am excited to see what he does!
My friend's horse has a really sensitive tongue. We tried a few different bits, all fully rubber like nathes. We found what worked best was going bitless. She's been a dream to ride since, with far less head tossing, resulting in a looser back, and a happier horse all around.