Dear prithvi by now you should have noticed that your father is ill and is suffering form a psychiatric illness called Senile Psychosis
. In this condition every thing around him looks suspicious and you have to remember that the experience of suspecting others is actually real to him because his mind is no longer able to differentiate from reality and imagined world that he is living in. This usually occurs due to excessive production of a neuro chemical called Dopamine
. hence he needs medication urgently some times in the thought of unfaithfulness he may try to attempt suicide
hence i advice you consult a psychiatrist
near your place about his condition get medication for him and put in his food with out his knowledge if does not like to visit a psychiatrist. We also have to evaluate your father for any tumors in the brain hence a CT scan
would be good idea, along with tests to make sure he does not suffer form dementia(forgetfulness).