It should improve in time, but probably not back to exactly how he originally was prior to treatments. Many OC patients will continue to have long lasting issues with dry mouth. You will see them always carrying a water bottle everywhere they go.

A complete recovery can take every bit of 2 full years. This is mostly having to do with sense of taste, fatigue and dry mouth. Recently a long time survivor (8 years post rads) reported she just had a big increase in saliva. She said she was undergoing hyperbaric oxygen treatments (HBO) for another condition and one of the side effects was her saliva is now functioning like it had before rads.

Since your father is only 2 months post rads, expect to see many improvements over the next few weeks and months. Its important to keep his calories and hydration up. Every day for at least the next year he needs a minimum of 2500+ calories and 48-64 oz of water. OC treatments are hard to bounce back from, with proper intake it will make everything easier. Hang in there, the worst is behind your father.