Can I give medicine? Current condition?

My father is suffering from delusion of infedility towards my mother. We are all suffering due to this from last 3-4 years. My mother is very very sad due to this. He tells us that she had an extra-marital affair with neighbors and had multiple relationships in past. She is unfaithful. I want to convince him that it is just his delusion but he didn't listen and don't want to go for psychiatrist consultation. I am trying very hard to convince him from years but of no use. Now, I have decided to give him medicine secretly in his food & after a month if he gets better then I will convince him to go to the doctor. Are there any tasteless medication available which will improve his current condition??? Please tell me. Please.