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.
He's a bit on the autism spectrum, which is why he's such a stickler to the ways he learned to do things. With him it's always about learning it the right way the first time because anything that comes after that is a no go ... or a huge struggle (one of the very few reasons why we still keep him IEP active).
I am so thankful that DS3.5 is already a great speller and will never go for wrong spelling! At least with him I won't have to worry about any of this.
