I agree with the mamas who said potassium and calcium. I do NOT AGREED with the mama that said to give motrin...PLEASE DO NOT GIVE YOUR CHILD DRUGS all you will do then is mask the pain and what you really want to do is find out WHY he is cramping not just hide the fact that he is.
Leg cramps could truly be anything, most likely it is something simple like low levels of potassium or calcium (and I don't mean milk). Bananas are a great and easy but a great calcium, well you have to go looking for it. My family (kids and all) have one main source of calcium. It is a supplement in powder form that we make into slushies or smoothies or milkshakes - yes we add it to soy milk for that extra boost. As I sell it, I am unable to post it here, but please feel free to send me a personal email if you are interested.
As for right-now-quick relief pushing on his toes, like he is standing on his tippy toes may help and leg massage, things need to keep moving to release pain. Cramping is about stuck energy, rubbing it in one direction will open up the flow.
Family Success Coach