You write "He has a little flap of skin on his bars that "pinches" into his
teeth with most bits......"
It may help for you to get your equine dentist to put a "bit seat" on his
first molar. This effectively keeps the skin from being pinched between the
bit and the sharp corner of the tooth. All the dentist does is is file away
the sharp portion so that a smooth, round surface is left. this has helped my
mare who likes to get behind the bit too.
Hope this helps.
Donna in Altoona