I think SWB explained it on the pinned WWS thread. It's a long thread but I remember reading it somewhere in there. You have to dig it up going through the pages. I meant to print it out but didn't do it.
We love D’Aularies’ Greek Myths here. I read to my kids when they were 5 and 6, and they (7 and 8 now) read it to themselves every now and then. We love all D’Aulares’ books and have most of them.
I don't think you are behind. I would keep working with him and spend as much time as he needs on the foundamental concepts and skills. He may surprise you later.