D.B.
You could go to the post office and get him a little stamp collecting book (there's a cool one now for dog and cat stamps), help him paste a stamp in the book and write the date, and as years go by on momentous occasions (first lost tooth, completion of each grade, etc.), he can add a stamp and enter the date. By the time he is an old geezer his little uncancelled stamps will be valuable to collectors, but mostly it is cheap and fun.