How about buying him a kit to build his own treehouse? You can look for various treehouse kits online as they come in different sizes and levels of difficulty. You can also gift him a book with ideas on unique and creative approaches to treehouse building, like "The Complete Guide to Treehouses" by Philip Schmidt. Alternatively, you can plan a weekend trip to a treehouse resort, where he can explore and learn about different treehouse structures while enjoying a relaxing getaway.
Or you can get him a star map with the date when he was born.
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.