Just learning to canter. He's been catered in a field less then a handful of times by previous owner. I have just started cantering under tack. I am a off the leg rider. I am not handy. Asked him to canter he automatically opens his mouth... but...does NOT grab and go just opens his mouth. What is this? What can I do to help? Rides in a D ring snaffle.
IMO, mouth-opening usually stems from tension, so it could be that he is tensing up from getting ready to canter. It's a new thing, his nerves are showing up.
Personally I like to just give them some time to figure things out before putting on a flash, etc. and giving him something else to fight against. Keep riding him and focus on getting him relaxed. When cantering is no big deal, many times the mouth thing stops.
If not, whip out the flash at home and the hunter crank at shows.
You might also try a french link. He could be reacting negatively to the nutcracker of the D ring if it is a single-joint hitting him in the roof of the mouth. Most horses I've ridden seem to prefer a french link to a single joint. Or maybe a mullen/HS Duo -- love that for my TB.
Make sure you are keeping the reins loose when you start cantering. I don't really loop them but I don't have much contact either, reins are just short enough to interfere if I need to (in case bucking or steering issues arise).
What sort of D-ring snaffle? He could also be reacting to the bit. On the track, one of my Thoroughbreds (whom I groomed) wore a figure-8 to keep his mouth shut when he galloped, and wore a tongue-tie on race day. Always gaped, including when I started riding him (even on a loose rein, I have very soft hands and at that stage also kept them open). I put him in this right away, and his mouth closed. Cracked a little sometimes but due to tension (took a long time to eliminate tension in this horse, he was perpetually tense), so very minor and it has gradually disappeared. I have him in something similar now.
Single-jointed will be pinching the tongue and exerting palate action. Double-jointed would be better, low port would also be another option to try (choose the port based on his mouth conformation but typically under 2'' and it should not interfere with the palate but will offer the tongue relief he may be seeking). Little more bar pressure but offers tongue relief.
I wouldn't jump to a crank or flash...he's got his mouth open for a reason.
....horses should be trained in such a way that they not only love their riders, but look forward to the time they are with them.
~ Xenophon, 350 B.C.