How about a subscription to a non-fiction book club? There are plenty of book clubs that focus on non-fiction books and dedicate themselves to sending out the best and latest releases. This way, your dad will receive a new book every month or two that he will be excited to read. You could also get him a book light to make it easy to read, even in low lighting situations. If he loves a specific author, you could get him a collection of their books, or consider buying one of the non-fiction books he has been wanting to read. Lastly, a Kindle would be an excellent gift that will let him carry around all his favorite books with ease.
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.