Could be he has a low palate. I have a horse that has a low palate, regular snaffles poke the roof of his mouth, so I use a jointed snaffle that has curved bars for tongue relief. Also, if you are pulling straight back with both hands, it causes a nut-cracker effect, so you have to be sure to NOT do that in a jointed snaffle, regardless. They are designed for one rein or the other to be pulled, curb or solid mouth pieces don't have that pinching force, and could be why you have less trouble with that. Bits with the port, if you have shanks on them, can still poke the roof of the mouth when reins are pulled back, but without the squeezing, as well.