If I had the opportunity to interview you, I would want to know if this is a new behavior or if he has always been a suspicious-minded person. If it is the latter, then it may be his personality. If this is a new behavior, then it may indicate a potential medical or mental health problem. Individuals with dementia-related disorders can develop paranoia as part of their illness. His behavior may be consistent with the development of a dementia-related condition.
Encourage your father to have an evaluation. If he is already under the care of medical professionals, report your concerns. Perhaps they can evaluate his condition. After an evaluation, the treating professionals can recommend a course of action. If a problem is present, medications can assist in reducing paranoia symptoms. Ensuring that he is evaluated and working with his treatment team will be your best course of action. Good luck and please take care.
Dr. Kristina Randle