Does he like chocolate or not? What are his favorite flavors and foods? That should help with the cake recipe.
What’s his favorite toy or movie or television show? Does he like sports? Nature? whatever? That can give you an idea for the decoration.
I know what my brother would have liked at age 5 (many years ago now!)—a plain, very chocolate cake with chocolate frosting. Nothing too fussy for the decorations. A model or picture of an automobile or train, maybe. (He grew up to be a precision mechanic and metalworker who works for a race-car driver.) No nuts because he’s allergic. (Does this kid have any allergies?)