I think you are right not to be concerned as it depends on each individual child as to whether they like to do things in their own creative way or easily get tied into the convention of following the rules. Enjoy the scribbles! Free expression and experimentation at preschool age is such a short phase and teaches them so much more than following someone else's idea of a picture

Don't encourage him to do anything more the experiment and have fun as what he is doing right now is setting up all the foundation for writing (and creativity) later down the track.