Herbs to Dry Excess Saliva for Brain-Injured Child

My son has had a brain injury and has tried many drugs with out success to help dry his mouth up. He swallows, but not enough naturally to keep saliva from building up in his mouth. Can you suggest an herb that might dry the mouth out a little? Preferable one I can get to a liquid since he can not swallow large items.

Thomas Bartram, in "Bartram's Encyclopedia of Herbal Medicine" (available at Richters) notes that sage tea (Salvia officinalis, or garden sage) is a known corrective for excess saliva. I suggest that you try the tea just as a mouthwash.
Bartram also lists Datura stramonium, 5 to 30 drops, as a corrective. Datura can only be prescribed by a qualified health practitioner because of fatal effects if given in excess. Datura contains the powerful alkaloids hyosine, hyoscyamine and atropine. It is contraindicated in heart failure, abnormal irritability, prostatitis, pregnancy and anti-depressant drugs.