My husband asks for the car keys every 15 minutes.

give him a set of keys that won't start the car..but looks like his or keys can be filed to make them unusable..and hopfully he wouldn't notice a very small nick in the key.. maybe this would help him feel in control of himself..let me know if this works for you..thanks

Give him a set of car keys that don't work. Then say, "oh my, I call for service to see what's wrong with car". Maybe just having the keys will be enough to satisfy him?

Can u distract him by saying "let's call first and see if he's home before you drive up there?"

Would it work to tell him that grandpa is on his way to your house? Let's look at those pictures of the grandkids to show him when he gets here. Let's have lunch while we wait. Let's pick up the living room before he gets here. Let's sweep off the patio so we can sit out there for our visit? What have you already tried?
I wish I had the perfect answer.

Have you got time to take him for a drive? Talk about granddad, see if you can lead the conversation back to present day, and stop off somewhere for a soda or something. Bit of luck he'll forget what you set off for - heaven knows I often do.

Alternatively, tell him you're pretty sure grandpa's away. Is there someone you could ring for him to "confirm" this information with?

Sorry you're having this kind of Sunday :(

He wants to go up on the hill to see about his granddad, who has been dead for years.

Where is it he wants to go? Perhaps if you can get to the bottom of that you will have a better chance of coming up with a diversion that satisfies him.