I honestly can't say if this is the reason, I'm just going on our son. My, now 16-year-old, son had horrible leg cramps as well. He's always been tall for his age. He's had the leg cramps when he was younger, but they really hit him hard in 6th/7th grade. By the time he turned 13 he was 6' tall. At one point he had grown 3 1/2 inches in 2 and 1/2 months. We had to take him to the doctors (for something not related to the leg pains) where they took his measurements, then we came back for a follow-up and the nurse was quite shocked and remeasured him.
He used to have a hard time learning how to control his body, since he grew so quick (think back to when you're pregnant and you still think you can fit behind a chair, but realize that no, the belly has grown, or your not as stable as you once were). He was kind of klutzy. His feet grew so much the poor kid had a hard time walking down the stairs. I used to think he was messing around when he would slip down the stairs. It wasn't until I had him put his foot on the stair that I realized half the ball of his foot and his toes hang over.
His leg cramps seem to have tapered off and he doesn't appear to be growing in height anymore, I think he was an early bloomer, but he grew fast. We just chalked it up to growing too fast. Again, I wouldn't count on that being medically correct, it was just our theory. When he would get the cramps he discovered it helped if I kneeded his calves. That's where the pain was the worst. His thighs just ached and we would give him advil for that.
Sorry I wasn't more help, hope that you get some great ideas for him.