Although 'Kumon maths' is so famous, and the founder of the Institute, Toru Kumon, was a maths teacher, Kumon is not just about maths. It is an approach to learning which is meant to help children with all subjects.
Kumon lessons are an addition to regular lessons rather than a replacement, so your child would still be expected to learn maths at school in the usual way. In Kumon lessons, rather than learning as a group, the children get a lot of individual attention and progress from one level to the next only when they are ready. So it is ideal for a child who struggles to keep up with the class. There is also regular testing - could be a problem if your child feels over-tested already, but the atmosphere is very supportive and you can keep re-taking, so not too much pressure. There is a lot of repetition which can get boring, but to be honest this is, in the end, an effective way to learn.
So it's probably worth a try at least - you can find one review here.