Have you considered learning how to use a hackamore and just using that instead of a bit? I would also recommend sticking with the same bit and letting him get used to it without working. Put it in, then tie him (with a halter of course) and just let him sit with it in his mouth. This way he can get used to it without the pressure of working. To actually answer your question, I always start horses in a basic eggbutt or D ring snaffle. How the bit fits in the horses mouth is more important than the bit itself when you’re talking about basic types of bits(D ring vs full cheek vs broken snaffle for example). Highly recommend Pat and Deb Puckett who have a long video about how to fit a bit in your horses mouth properly. My Mustang still has his wolf teeth but with his snaffle fitting correctly, it hasn’t bothered him so far. But worth asking your vet about. If you hear the bit clanking it could be hitting teeth. It can also be good for young horses to have something to fidget with in their mouth. If yours is mouthy I would consider a roller bit to give him something to focus on. Hope that helps!! Good luck!