I would definitely follow through with the appointment, but I remember my legs feeling that way (and my back, and my arms) when I was growing really fast. It wasn't all the time, it would come and go. I even developed these lumps under my knees because the ligaments were so abused from being stretched like crazy. My mom used to give me children's aspirin for it and that did help, as did a hot water bottle on my knees. (By the way, I grew from 4'6" to 6' in under 3 years, so I hurt pretty much all the time!)
Is your son hungrier than usual? When my kids are about to start growing, they eat like it's going out of style. Also I have noticed that they tend to get a little thicker around the middle right before they grow, like their bodies are storing the calories in order to grow. If you have noticed any of these signs, then maybe he is just starting to shoot up in a growth spurt right now.