You know, no amount of reassurance will help here. Just help him. The sooner you find this service for him and choose a monument together with him, the better it will be for his mental state. For example, I know that granite monuments https://thga.us/ operates 24/7, and their website is also user-friendly. Your grandfather will be able to see what gravestones are available, make a preliminary calculation, and get back to sleep with confidence in the quality of service. By the way, if he's thinking about monuments, he'll live at least another 5 years. I went through the same thing with my grandmother.
But the main thing is that my sister is happy, so I'm willing to put up with it.