Why do you want him to focus on creative writing over other types of writing? Does he have a strong foundation in writing already? I have found that students who are NOT natural creative writers tend to do better with creative writing when they have done work in how to form a paragraph/how to brainstorm/outline/and have seen it modeled.
I agree with separating spelling from writing - there are a number of good spelling programs out there.
Do you want to use a writing curriculum? There are a number of good ones that go step by step in writing.