It seems like with the summer school for writing, what you've got to work on mostly is math, am I right?
Math is hidden all over the place. There are things you can do, that he might not even know is him studying math! Like baking. Make muffins together, and double the recipe - you are adding fractions. Have him go to the grocery store with you, and let him keep a tally of how much money you are spending as you put things in the basket (which is a good money-saving exercise, anyway!). Now you are rounding (up to the nearest half dollar, maybe?), and doing addition with decimals. Does he have an allowance, or at least a piggy bank? Figuring out how much money he has, and how much he wants to spend, and how much he'll have left over, is math, too.
We have enjoyed the Kumon workbooks, too - they are available at a regular bookstore. There are some for lots of grade levels and subjects. Also, duck into a teacher supply store, and explain the same thing to them, that you have told us. They will probably have lots of helpful ideas!
Mostly, though - try to have fun with it. Do something YOU find interesting, too, and that will speak a lot to him. He'll have more fun with it, if you are having fun with it, too.
I hope this helps a little bit, anyway. Have a great summer!