Make sure that the bit actually fits him. His teeth could be fine, but the bit could be the wrong type for his mouth, or too big or small. You may need to experiment with a few different kinds - for instance, some horses do better with more jointed bits like a French Link.
I would not have him eat with the bit in, or leave him unattended with it, bad idea!
Also, be sure if you are lunging him with the bridle on to either take the reins off, or secure them. You don't want him getting tangled up/stepping on them.
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