Let's take the sertraline out of the equation for a minute. If he is taking other drugs in a "cocktail" format, they could be creating an interaction effect that would affect dopamine levels. Combined with his advancing age, the number of combinations is endless, and takes me out of my knowledge base. My recommendation to you would be to gather his records/prescriptions from all of his doctors and present them to the doctor with whom you feel most comfortable or with whom he has the best relationship. Too, s/he should be familiar with the changing neurochemistry of the aging brain. It will be that physician's responsibility then to trouble-shoot the potential interaction effects.
But it bears repeating, the sertraline will not impact the systems that would be responsible for the paranoia and delusions. I would include it in the list given to his doctor, but invest more energy in examining other interactions.
It is also possible (unfortunately) that your father is suffering from advancing dementia
. At his age, it would not be common, but not impossible, either. A positive family history for Alzheimer
's or dementia would raise a red flag, as would if your father has been forgetting little things for the past few years. Still, to be this floridly paranoid would discount an acute onset - in other words, he would not get to this point quickly. His condition would have deteriorated slowly over the course of a few years. A fast onset almost invariably points to medications.
A geriatric psychiatrist would be the source for this knowledge.