After signing up J for preschool, I had a revelation that I had not spent nearly as much time on letters and numbers with him as I did with his sister. (Poor second child.) After working with him for a few minutes, it was determined that he really only knew the number 1 and the letter O. So apparently he’s all set to write in binary.
(After some diligent work he can now recognize the letters in his name. There is an O in it, after all.)