I am sure that your DS is just suffering from growing, however, have you noticed any of the following whilst he has been growing up
1)Joints easily dislocate or seem weak (i.e. ankles give way for no reason).
2) Bruises/Scars easily.
3) Joints are very loose or can be bent in unusuals ways.
The reason I ask is that my DD suffers from a condition called Hypermobilty Syndrome and this can result in very bad "growing pains" particularly at night. My DD has been troubled with pains in her knees, ankles and wrists since she has been about 18 months. (She is now 6 and she still gets the pains). The health visitor told me that it was just growing pains, Jo-Ann then dislocated her elbow three times, her wrist once and got "whiplash" when playing with her cousin. I then decided that this need to be checked out. We also found out that when she started school that she could not sit with her legs crossed as this cause her hip joint to "pop out" and she started to get pains in her fingers once she started to write. It is worse when the weather is cold.
Hypermobility Syndrome is basically a condition that effects the collagen that supports the joint nad is found in skin. In people the have HS their colagen is over stretchy and provides no protection for the joints so the muscles have work harder to try to stop the joint from being overstretched. This the leads the muscular/cramping pains in an around the joints. Their skin scars more easily and does not heal as quick. It can also lead to problems with the bowels resulting in constipation.
In most cases no treament is needed just a little more care when taking exercise and staying away from any activty that you know make sit worse. Also lots of Paracetemol and Ibuprofen when the pain is really bad.
It may sound terrible but my DD is quite proud of the fact that she can lick her own elbow, even though it is supposed to be impossible.
As I said at the beginning it is most probably just growing pains, epesically if you have not had the problem whilst he was younger, but I think that people need to be aware that it is not always growing pains.
If anyone wants to know more please follow this link to the HMSA website.
http://www.hypermobility.org/whatishms.php