It sounds to me like you've gotten as much out of WWE as you're going to get right now, and Writing Tales is fantastic, so I wouldn't feel bad about switching. That's not curriculum hopping, it's moving on.
My 4th grader is about halfway through WT2 and my 3rd grader is just starting WT1. I would probably start with WT1 in your situation. I think the shorter stories in WT1 are great for a natural writer, where the focus can be on improving the quality of the writing, not just on getting it done. You'll probably want to speed through or skip some grammar sections, and he'll get through each level in less than 30 weeks. I love WT so much that I'd want to get as much time out of it as possible. :)