We have this problem a bit as well, in that my DS4 (almost 5) is probably low-average on handwriting. He likes to "do math," but he still can't write a 3 reliably and reverses a ton of numbers. Also, the spaces in workbooks at his level are too small for his writing.
We do a lot of oral math, and sometimes use a whiteboard (lots of room--easy to erase). However, I am not homeschooling, so this is more casual.
Also, if you buy workbooks, you can go through them and looks for pages that can be done without much writing (mostly bubble-in).
