My 8 year old TB gelding chews on the bit almost immediately when I put his bridle on. We event at BN and he does this all the time, even when we are just hacking out an a sunny Sunday. I've had him in a loose ring French snaffle and then switched to an egg butt. I've tightened, loosened and even changed from a 5 and a half to a 5 and a quarter. He is a good horse, obedient, willing, responsive not anxious or nervous. I don't know if he does this out of boredom or what.
Is this a problem or should I just except who he is... a bit chewer?
If it has been longer than 6 months since his teeth have been done, you should probably have them floated, and have the vet or dentist (whoever is doing the teeth) check the gums and the mouth generally to look for sores.
The horse might also benefit from having a chiro look at him.
You could also try a baucher bit; a lot of horses that fuss with the bit prefer the baucher and have a quieter mouth with it. Another possibility would be a plastic mullen mouth, like the Nathe or HS Duo bits.
"The formula 'Two and two make five' is not without its attractions." --Dostoevsky