It is most likely his mouth is irritated by the bit. Tongue could be numb. The horse keeps trying to get relief by constantly moving his mouth. It is not normal for horses to stand around constantly biting and chewing. They aren't cows.
Actually it is normal for a horse to salivate and play with the bits-it shows he is relaxed. If they stand with their mouth still and dry ait can be a sign of tension. I presume that you do not ride or have much to do with horses.
