Grab a copy of the curriculum your son would follow if he were in mainstream school and make sure that's your minimum target.

Aim for an outing at least once a week too - a park can be educational (I mean, schools do pe, right?) as can a museum. Sometimes you'll need the change of scene just to avoid going crazy! Also have a dedicated "sit down and work" place and time. Most things can be learnt through fun and cooking dinner with you is a lesson, not a chore, but sitting and actually doing some pen/paper work every day and in that routine, even if it doesn't seem much, is important.

I'm not a HSer, but I was HSed for some of my primary education and wish I could HS DS.