Sour secretions from mouth. Appreciated.

My father was diagnosed with tongue cancer last March and underwent a hemiglossectomy followed by 30 radiation sessions. It has now been six months since completing radiation. He recovered well overall and experienced heavy neck tightness during the first three months, which has improved but is still present. Our main concern now is that he has been having sour-tasting secretions in his mouth, which make all food taste sour. This has been going on for about three months with no improvement. His radiation oncologist hasn’t been able to determine the cause, and we even consulted a gastroenterologist to rule out acid reflux, but that wasn’t the issue. Any suggestions would be greatly appreciated. Thank you !