When I was growing up, I had such horrible pains in my calves and behind my knees, sometimes moving up to the tendon that attaches my inner thigh to my torso. :( It wasn't until early teens I learned they were indeed "growing pains" which I thought was either a silly t.v. show or a myth.
Besides hot baths, Tylenol and massages, there wasn't much taught to me to help. Even as a young high school student, I suffered. I brought it up to my P.E./English teacher and he told me to lie on my back with my tush up against the wall and my legs flat, straight against the wall for a count of about 30 seconds or so. Then, bend my knees for about 15 secs. and then repeat for about 20 minutes. This helps with circulation and pain. Also, most stores sell the Hyland's brand of homeopathic/natural remedies for all sorts of ailments, including leg cramps. This helped my girls. Try that.
My two girls have suffered as well, and pressing the legs against the wall as my teacher advised really did help a lot. Try not to have him run too much, play on monkey bars, and things like that.
Good luck. It's tough when there isn't much in our "parental arsenal" that can help. But, seriously, try the legs on the wall... it costs nothing and helps. My teacher said that with poor circulation, this "brings the bad blood from the feet to the rest of the body so it can be recirculated and sent back through". Whether it was poor circulation or growing pains, it helped!
Let us know what works for you guys!
L.