I just looked this up the other day myself because my 4 1/2 year old was complaining of leg pain behind her knee and saying how uncomfortable she was at bedtime. I had her stretch, I massaged it, and then I put a heating pad on it for a bit. The pain was gone the next morning, but she had it again the following night and since then has been fine. I had them growing up too and remembered how much they hurt and what my mom used to do to make me feel better.
Growing pains usually happen when kids are between the ages of 3 and 5 or 8 and 12. Most of the time the pain is in the front of the thighs, the calves, or behind the knees. Growing pains often start to ache right before bedtime or they might wake up in the middle of the night with their legs hurting. Growing pains go away by morning so if your son is in pain during the day or it continues to persist, it might be something to talk to his dr. about.