http://washhomeschool.org/advocacy/part-time-enrollment/ has info on WA State partial homeschooling.
You're going to need to file a declaration of intent to homeschool with your district and that's usually about it.
I'm going to assume you're in Vancouver in which case its here:
http://vansd.org/student-welfare-attendance/alternative-education/But I believe you can find the form on every school district's site .
That said, its a pain in the neck to do and if after-schooling is working you might consider having your son treat the math in school like review and just letting it be.
