What to do for a horse that opens his mouth/grinds bit?

This horse that I've had a few lessons on, a thoroughbred, is no spring chicken but not an old timer. He raced and did a little eventing. No one really rides him (some kids did and he tolerates them but I don't think they did much on him) and he use to belong to a farrier. I have very soft hands and everything is going well but I hear him grind his teeth sometimes when I ask him to go into the bit pressure. He goes in a happy mouth snaffle with one joint. Sometimes I see him in the mirror as Im troting by with his entire mouth opening. Will he ever get out of this habit? I will never tie his mouth shut with a flash. I've seen horses with a flash on that just grimace horribly with all their teeth showing.