Kristy he doesn't suck back or evade the waterford, it just makes it impossible for him to learn on it or get to deep which is his preferred method of evasion of the mullen mouth.
As a responsible user of a leverage bit I use a snaffle rein and a leverage rein on my xc bit so I only use the leverage when I need it.
Tell him about double jointed bits, he doesn't want anything to do with them. I've tried believe me,however if you've noticed the pattern with the bits none of them touch the roof of his mouth.
The horse chooses the bits.