In the end, it depends entirely on what the specific child would enjoy more. A math club offers access to other kids who love math, and potentially to experts who can share some of the beauty and joy in off-mainstream math concepts. If he's feeling a bit on the odd side at school, a club may be a great place to reassure him that his interests are OK and there are other kids "like him". On the other hand, if school is still a comfortable fit socially, he may not feel that kind of need now. Would he enjoy more scheduled, out-there time with other kids? Or more unscheduled free time at home?
DS (now 11) loves doing AoPS at home with me, but detests the idea of anything that feels like more school. At home, in contrast, he still sees it as a great chance for one-on-one mommy time, not homework. We can also binge and reduce depending on time available, homework, friends, and distraction by particle physics. And I can focus on what's important to us - concepts and problem solving - adapting to his needs by doing the writing myself on a whiteboard (he has writing issues), and skipping final computations if they are straightforward but time-consuming. I have eventually come to understand (and slowly accept) that I could never sit him down on his own with the book or an on-line course and expect him to do anything with it: what he values is our personal interaction. He's the antithesis of an independent learner - math by Socratic dialogue is more his style.
From my own experience at least, I can't see ever telling a kid there's something they aren't allowed to try to learn yet. If they're not ready to apply it, oh well, they've learned a cool concept and they'll grow into its use. You might find the AoPS pre-Algebra book (and/ or videos) a good reference for teaching miscellaneous concepts as they come up (like negative numbers or geometry) - it's basically an intro to everything (and covers all the prerequisites too, if you need them).
