A friend of mine said her son was assessed at one school at a 4c and then dropped to a 3a at his next school. From what she could reasonably work out, it seemed as though it was a combination of one school over-rating and the other being perhaps a little stringent. I'm sure a teacher will have something more useful to say on this, but I wonder if there's one particular area that he's a little wobbly on that's keeping him pegged at a low level, as well. These things are assessed on a framework, AFAIK, and it may be one component of that framework that's stopping him from moving up a level.
Your question was really about Kumon: we did Kumon with DS in KS1, so I'm not sure how it develops for slightly older kids but my feeling was that if you have serious catching up to do, Kumon will not get the job done. As another poster said upthread, it seemed to be based on repetition and practice; if you aren't getting high enough scores, you go back a level until you are at a level where you can work wtih perfection. Seems to me that perhaps your DS might need a more explanatory form of assistance.
Forgive me for asking, OP, but how much does your DS read? I do believe reading constantly and widely, a range of stuff (fiction and, for example, the sports pages of the newspaper, even comics or magazines), gradually nourishes an understanding of punctuation. It does take time, though.
Reading out loud, according to my DS's head teacher, helps in observing punctuation on the page and sentence structure because if you're reading out loud, it's harder to scan over stuff so you absorb it more completely. I believe this to be true, but again, it's a process that will take time. The reason I particularly suggest it is because this isn't an area in which you need to be particularly confident yourself to help him! All you need to do is make the time and keep quiet!
sorry for the long post, and hope you get both some reassurance and some help. I offer you baked goods. 