Lisa, if you go to the main site (click on the link above) you can read through the diet I recommend. However this is too strict for most people to follow on a regular basis (we are addicted to our sugar!). Kids do tend to drink a lot of soda and sweetened drinks. Even some bottled water now has potassium added. I have made skim milk my main beverage all my life and most of the people in my family with MC were milk drinkers, too.
He's really too young to be on the high-powered medications yet, but he may get some relief using the amino acid L-Taurine and the herb Licorice Root extract. These are both available at the health food store. Taurine acts like a sodium channel blocker similar to Mexitil, and the licorice root causes an increase in excretion of potassium similar to some of the diuretics that used to be prescribed for MC. One or two capsules of each per day should be adequate.
I have seen many younger patients do well with Acetazolamide or Diamox. It seems to have fewer side effects and is even given to young children with certain forms of periodic paralysis. You might tell his parents about it and have them ask the doctor. The hormones that spike during puberty tend to make MC much worse. He may have less problems as he gets older.