There could be many causes of it, I suggest seeing a dentist to find out, in the mean time, here are a few that can be done to help alleviate the problem:
Home Remedies For Excess Salivation
The excess saliva in the mouth may be reduced by sucking on a piece of lemon. This is known to regulate the quantity of saliva in the mouth. Avoid eating too many sugary foods as sugar stimulates an increased production of saliva. The excess saliva may also be absorbed by placing a small pinch of ground coffee beneath the tongue. Another good remedy for excessive saliva in the mouth is to suck on a few sunflower seeds. This is also a beneficial remedy for increased saliva production during pregnancy. If the condition continues for a longer period of time or becomes aggravated, it is advisable to undergo a medical examination as it could be indicative of problems within the salivary glands.
Hope this helps.
I typed this in on webmd.com this could be very serious. Please go to the web page. You know more of his symptoms and I wouldn't want to tell you something I wasn't sure of. It could be life saving though, so please read at webmd.com
Excessive salivation is an ocasional side effect of a proton-pump inhibiter that reduces stomach acid. It can also occur when something interferes with production of stomach acid. The mechanism seems to be that the body attempts to make up for the diminishing of digestive ability by providing additional saliva. So I would look for a cause that would stop acid production in the stomach. Of course that's only a possibility.