How can you get your horse to stop opening his mouth when you are riding him?

First of all you need to figure out WHY he is opening his mouth so much. Most of the time, it's pain or discomfort, usually from the bit.

First, check the fit of the bit, that it isn't too big or more imporatantly, too small.

Then, see if your horse opens his mouth with a snaffle. If he doesn't, then the bit was probably too harsh before and you should go back to a snaffle.

If you ride "with contact" most of the time, see if he opens his mouth if you ride on a loose rein in a snaffle. If he doesn't, then you need to check out Parelli's "Game of Contact" to teach your horse to willingly accept contact.

Also,a factor that people rarely take into consideration, is does the bit taste bad. Copper bits and "sweet iron" bits are very bitter. Taste it for yourself. Rubber bits also taste foul. So do your own tests to see what bit has no taste, or tastes good and try that.

Try a thinner bit. Most horses tongue fills up their mouth, leaving little room for a bit, therefore pressing the tongue down and shoving the bit into the roof of the mouth. So when you think about it, thinner bits, in good soft hands of course, are more humane than thicker ones.