Thank you for advise. I know he has a large tongue. He was going beautifully in quite a thick lozenge bit before teething but can't cope now. I'll have a look at the mylers.
I have a horse who since having his wolf teeth out and his tushes have come through no longer seems happy with a bit in his mouth. He has had X-rays and all his mouth and teeth look good. No abscesses or bits of tooth left in the gum. He tends to brace his lower jaw and then goes behind the leg...