Lucy, sorry for my poor English.
There are some drugs that produce dry mouth (xerostomia). They can bring some comfort to your dad. Besides the use of drugs, others ways can produce the same effect, like very localized radiation, but this is a permanet and irreversable treatment. Some advocate for cutting the innervation of the salivary gland, which will cause it to stop producing, but this may be severe treatments for a situation which may pass.
Yours sincerely
Pablo Gimenes Tavares DMD/PhD