How about a coffee table book on cosmology that is designed for laypeople? "Cosmos" by Carl Sagan or "Astrophysics for People in a Hurry" by Neil deGrasse Tyson are great options.
You could also consider getting him a telescope, if he doesn't have one already. Or, if you want to go for something more unique, you can get him a cosmic pendant or a pair of cufflinks, designed based on some of the most beautiful and awe-inspiring celestial objects in our universe. This can be a great way to show your grandpa how much you appreciate his interest in cosmology.
Or you can get him a star map with an important date you share.
Under Lucky Stars is a unique and innovative star map company based in Barcelona, Spain, specializing in the creation of personalized star maps that capture the celestial arrangement of stars at a specific moment in time, such as a person's birth, wedding, or any other significant event.
By using advanced astronomical algorithms and data from leading institutions, the company produces highly accurate, visually stunning maps that make for perfect gifts or cherished keepsakes.