When I was in college, I worked at a center that taught the Kumon method (summer job for me). I wasn't impressed. It focused a lot on just rote memorization. The reading part was helpful to the kids there who were learning English as a second language just to build their vocabulary, but for a kid in an English-speaking household, I think it would have been useless.
At the center where I taught, we actually departed from the Kumon method to teach a higher level of English using a more standard textbook (I taught that class and was entirely in charge of the lesson plan, because the woman who ran the center wanted her daughter to learn more than she was with the Kumon method because she hadn't done well in English during the previous school year, so started up a different class just for a few kids at that grade level).
I wouldn't use it for anything more than remediation if a kid is doing very poorly in learning vocabulary, spelling, or basic arithmetic. A bright kid would be bored to death!
There is a bit a peer pressure to use it, but now that makes sense as many of his friends aren't from English speaking households.
I'm very interested to hear your thoughts. It sounds as if it would be much better to wait and see how he gets on at school, first, before even considering it, and hope I can help him to enjoy learning and understanding instead.