If you could get a kid to actively participate in the school and the community it would help them a lot I bet.
I have been looking at Introduction and intermediate to counting and probability for my ds 10. He is pretty stretched right now but maybe for some summer work. It is pretty cool stuff that could get a kid hooked if they hang in there but might be too much review.
Does he do Alcumus, and if so, does he have a high level and all topics blue in the counting and probability there? That's the obvious thing to use as a guide. The intro c&p is reputed to be the easiest intro course, but it looks to me as though the intermediate requires a level of mathematical sophistication that makes it unsuitable as an immediate follow-on from the intro course. For example, it does induction half way through, something which (for reasons I've never fully understood!) many people find to be a conceptual roadblock.
