It's probably because the bit hurts your horse. Bits create pain on the palate and your horse tries to release pain by putting his tongue between his palate and the bit. Try to ride him bitless.
It's suppose to stop your horse from playing with the bit and for those horses that accidentally bite there tongue while riding
Try a bitless bridle. Bits Hirt horses. It is on Internet.
First of all, make sure that your bridle fits your horse. Maybe that's because your hands hurt him, be softer and don't tighten too much your noseband, to see :)
Marlee, I don't have! In which way would it be useful?
Maybe the bit doesn't fits him and he feels uncomfortable with it, or maybe a dentist problem!
do you have a flash band?