At 6 social skills are far more important than math. You can do fun, easy, math projects at home this summer and he will catch up fast. There are lots of m&m, oreo, etc.... math books and honestly, kids learn math a LOT faster and better from that than from worksheets.
Our school was flabbergasted when I calledWiz in sick one day because he had been up until4 figuring out multiplication. He was in first grade. He was SHOCKED when the idiots told him he wasn't old enough to do multiplication. So was I.
As long as you provide some help at home,he is young enough that it won't be a big deal IF they have stuff for him to learn on social stuff.