Reddit mentions of Scaredy Squirrel

Might be time to have a chat with him about worries, and how it's good to be safe but it's not good to be so worried all the time that it gets in the way of having fun (like sleepovers at grandma's). You could read him a book like Scaredy Squirrel or Wemberly Worried and talk about what the character's worries are vs. what his own worries are, and how worrying doesn't really help either of them.

One strategy you might try is having a "worry time" every day, setting a timer for 5 minutes where he can talk and get all his worries out, and then when worry thoughts come up at other times you can say, let's save that for worry time. You could also give him some worry dolls (or a worry stone, or whatever) to "hold" his worries for him - someone once posted on here that their kid had a Darth Vader toy that he gave all his worries to every night before bed! You might also find some helpful ideas on this site or check out the book What to Do When You Worry Too Much.

You could also try reading a book like Lifetimes, which shows death as part of the natural cycle of life, in the context of animals, plants, etc. You want to normalize the fact that yes, everything that is alive will eventually die, but most people don't die until they are old and have lived a nice long life and that's not something he needs to be worried about as a six-year-old. You could also have his doctor reinforce this next time he goes in for a checkup - that he's healthy and doesn't have to worry about dying anytime soon.

(edited to fix link)