Something we do, which is a little old fashioned, is let horse wear bit and bridle in his stall. We start with short sessions of a hour, then build up time. He has no food in the stall at first, but it will be in there later. In my experience it was called “mouthing” the horse, everyone dId it in some form or another when I was a kid.
Loose ring jointed snaffle with rubber bit guards to prevent lip skin pinches as ring rotate. Fitted a bit low in the mouth to allow horse plenty of room to play with the bit. I would have a throatlatch on the bridle to keep it on in case he rubs. No cavesson or nosebands, you WANT him opening and closing mouth, getting tongue over and fixing his problem HIMSELF. No people getting excited, though you can rescue him after a little time, when he can’t fix tongue himself. This is why bit is fitted loose, not making him “smile” after bridling. Most figure out that tongue over is uncomfortable, quit doing it.
We increase the wearing time and start feeding horse with the bit on, grain first, then small amounts of hay. He always has a bucket of water. For our horses, they play with bit a while, then over time it loses the excitement factor. They learn to manage the food, chewing, drinks while wearing the bit. No excitement when getting bridled anymore. It is NOT a clue to getting worked or doing new things.
It seems to help a great deal with our need to sponge bridled mouths during use, not grabbing reins, helps them stay calm when brought out to be used. They quit gnawing on the bit after bridling, after spending their stall time with bridle on. I believe it is like old chewing gum, their jaw just gets tired of chewing after while! Ha ha
You will probably need to put your thumb in mouth corner to get him to open mouth when unbridling. Mine seem not to want to let go of bit and I don’t want to clank the bit on teeth when removing bridle. Often using the thumb and “open” word get them trained to help you by opening their mouth on command.
I do think some horses in families, have a need to chew, it calms them. We had a mare that would ALWAYS chew when handled, standing or under tack standing. Tacked up she would stand all day clinking her loose-jaw bit. Lift and drop the mouthpiece. Give her the chain shank to mouth when shoeing, never moved. One of her the sons was just like her. In fact he got her bit after her retirement and loved it! The other two boys went thru mouthing school never had an issue chewing/playing with things or their bits.