Yes, I agree with khanacademy as a great starting point for math. Depending on how weak the basic math concepts are, the practice provided there at the early levels may not be extensive and varied enough. But even so, it's a great way of figuring out which skills and concepts are already solid, which can be quickly brought up to scratch, and, if necessary, what level it might be good to jump into with another program. If you need another program, Singapore Primary Math is relatively inexpensive, has a good conceptual base, and can be worked through fairly quickly by kids who are developmentally ready to "catch up." I would think that starting at about the 3A level might be good, since that's where the bar-diagram approach to pre-algebraic problem-solving is introduced.
My dysgraphic ds started school (after unschooling all along) in grade 10 and the preceding spring we requested formal assessment by the school district in order to have accommodations organized before they became an issue. We had it in writing that for any assignment or assessment which required more than two to three sentences of writing he should be allowed to use a laptop. His keyboarding skills were already very good; if they hadn't been, we would have prioritized them during the previous year with something like Mavis Beacon.
We chose to have him use his own laptop rather than relying on the school district to provide one. The disctrict would have been required to do so, based on his formal identification, but he had already saved up enough money to pay half, and we parents kicked in the other half just to simplify things and to give him confidence that the hardware would meet his needs, and he could have full control over installing whatever software he wanted. Also, our district has so few laptops that they often have to be shared, and that means students are having to go retrieve them from the office when they are needed, they can't necessarily be taken home, and batteries are often not charged. My ds was worried that he'd feel really self-conscious having to request permission to leave in the middle of class to sign out the laptop, and then sit in a specific spot so he could plug in, and so on. Far easier just to have his own unit in his backpack where a note-taking binder might otherwise be. So if you don't already own a laptop that your child could use for school, you might give some thought to budgeting for one, if you think that solution would be simplest.
Unlike transpecos' experience, my kids' experiences in school have largely been very good. None of us expected school to replace unschooling: it was just a way of meeting certain specific desires for structure and accountability. We were fortunate to find flexible, caring people who appreciated the unusual educational paths my kids had travelled, and continued to travel in their out-of-school time.
Miranda