How about getting him a field guide for plants, trees, and wildlife that can be found on nature walks? There are many great options available for kids, such as The Kids' Guide to Birds of Minnesota or Peterson First Guide to Reptiles and Amphibians. Another idea is to get him a nature journal where he can record his observations and sketch what he sees on his walks. You could also consider buying him a pair of binoculars, a magnifying glass, or even a small microscope to help him observe nature in greater detail.
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.