Well, if you're looking to make the skip over the summer, you have a few more months before he'd have to take the exam, assuming they'd do it not long before the end of the school year. I don't see anything wrong with teaching him a few math concepts that he hasn't been exposed to yet, if he's interested. That's one aspect that has always troubled me - how a person, no matter how gifted, could have an achievement level that included things they had never had the opportunity to learn about. If there hasn't been an opportunity at school, then there would need to be an opportunity at home.
I have no advice on acceleration but I'd love the link to the math exam you found!
