My horse chews non stop on his bit. Is there anything I can do to prevent this?

In many ways it is looked upon as it being a good thing for a horse to chew on the bit. A bit can work like a pacifier to keep your horse calm. Now if it is excessive then many times it is because the bit does not fit right. You may have your bridle too tight and it pulling on his cheeks causing him discomfort or it is too loose and and so he is trying to adjust it with his tongue. Most of the time you want it so you bit pulls back on the back of his mouth to where it just starts to cause a crease at the back.

Also it is beneficial to have your equine dentist file his teeth so he has a bit set in place. This way the bit will set in the correct place in his mouth.

The type of bit makes a difference too. If you are using too harse of bit for the level of training that your horse knows then he will allows be focusing on the bit and working with it to avoid feeling pain from it. I personally like using a snaffle or a very mild curb bit that breaks in 2 places for the mouthpiece with a copper roller in the middle.

Trying these things may work but there is the possibility that your horse has made this a learned behavior and so will always chew it excesively. One cure for that is to teach your horse to not use a bit and to be able to ride bridleless :) I personally enjoying doing most of my riding in just a rope halter.