Originally Posted by jovi_y2k2
Has he had his teeth checked recently? This would be my first course of action. If all ok there and he's still doing it I'd be looking at the confirmation of his mouth and the type of bit you're using. For example if he has a fleshy mouth with a big tongue and a thick bit, there won't be much room in his mouth to accommodate the bit, so might be unhappy about it hence the chewing.
Open the mouth and chewing the bit are signs of unhappniess and should not be sorted by putting a flash on. Go back a couple of steps and try and find bit that he is happy with (use a bit bank as it can be a long expensive process) it took me 7 bits before I found on ehe liked.
It will also depend on the rider and how they have the contact a sloppy contact will result in opening mouth and chewing. Also a incorrectly fitted but can gve to much movement. You could try round cheek pieces if you find a bit that he likes but moves around side to side to much and helps with steering.