Inventive spelling is a developmental thing. Once he becomes a strong reader and sees that he is misspelling words, he'd start caring and would learn to spell correctly. He's ask you over and over "How do you spell..." and you'd wish that he stayed on the inventive spelling stage a bit longer. If the transition never happens naturally in then next year or so, then you might want to put him on a structured spelling program but I personally won't worry about it right now.