Given your grandpa's age, delusion is quite a natural pattern to be observed in such old age people. In order to be able to get him to a doctor, you can, first, try winning his friendship for yourself. If you are his friend/confidant, who doesn't question his beliefs/thoughts then he'd be able to trust you. Once you've gained his trust, you can introduce him to a doctor. If the doctor/psychiatrist/psychologist can be requested to visit him than him made to visit, it might open up a way forward.
Next Steps
First step is to be friendly and non-judgmental towards your grandpa. If you are unable to be in close proximity of your grandpa then you can seek help of that one person whom he speaks often to or seeks the help of.