OP here. Thanks so much for this feedback! I was leaning towards Singapore Math based on the recommendations of a math teacher, but it sounds like there may be better options for us out there. I need little to no prep, some online work (at my son's request) and common core. Engage NY was another site my friend recommended, but I am overwhelmed by it (looks like a lot of work). I'll look at Khan. My son is a pretty engaged, quick learner. We're pulling him out of school because he's testing 3-4 grades ahead, and needs more challenge and to move at his own pace. There are still lots of basic math concepts that he hasn't been introduced to.
Anything else that jumps out at you as a possibility for us?