I'd say you keep going with the math at home. The math was one of the reasons why we're homeschooling our DS7 too. Last year, in kindergarten DS was only do addition yet in pre-k he was doing multiplication. We ran into all sorts of headaches with the school.
This year, he's rapidly accelerating through curriculums/grades. I bought a very cheap general curriculum series (grades 1-6) to make sure we're covering the basics; I didn't spend more than $25 for the entire series. But DS is doing more online - much of which is freely available and accessible - and also through books from the public libraries. So I think you'll find the situation will get somewhat easier for you when you start homeschooling next year; you won't have to deal with the school's policies. When you homeschool, you can jump around or decimals in a day or two.
Once a child has finished 6th grade, the situation changes as others have mentioned. Then it's pre-algebra, algebra, geometry, calculus, etc. There's AoPS, Thinkwell, Elements of Math, Living Math, LoF, Khan, Education Portal, etc. So the world becomes your oyster.
In the meantime, I would let him read math books like the Murderous Maths series, Life of Fred, and anything else I could get my hands on for free or cheaply. I'd also maybe start looking at various mathy things online (i.e. xtramath.org or something on fractals or anything that tickles your son). Go to Hoagies Gifted Education; they've got a ton of resources for math.
