Which would you pick

We have always called my son by a nickname. He will be starting school in the fall, and I want to label his items (backpack, lunch box, etc). But now I don't know whether to label them with his birth name or nickname. The school said they don't mind either name. They just stated they would teach him to write his birth name. But now my son prefers his nickname and says his birthname is not his name. He is 3.5 I am leaning towards his birth name so he can get used to it and will be a visual reminder whenever he sees his bags. We love his nickname, but it has seem to have complicated things now that he is starting school. 😅