I encourage you to get a second opinion. Although it is probably simply "growing pains", it could be a symptom of something more serious. I'd try to figure out, with his help, if the pain is in a muscle or in a joint. He's old enough to show you where it hurts and to describe it to you in his own words.
If it is a muscle cramp, then water, bananas, gatoraid, etc, should help. If it is joint pain, then get the specific joint or joints checked out by a pediatric orthopedic specialist. It is is deep bone pain, that needs to be checked out too. Even small children can suffer from bone and joint problems which respond well to treatment. It's not likely, but still possible, that he could even have a fracture or partial fracture. There are many things that can cause muscle cramps, but if this is persistent than a blood workup to check for various other medical causes is in order.(Things like a thyroid disorder, diabetes, or parathyroid disorder to name a few.)
Finally, if the pain is associated with severe cramping of the calves, if he has very well defined calf muscles, consider have him checked for muscular dystrophy. Although it usually is considered a hereditary disease, my sister's son has MD with no familial history of the disease. Usually MD is diagnosed in while boys are very young, however when there is no family history of MD it can be overlooked. My nephew had problems similar to the very short description you gave from a young age, but was not diagnosed until he had already begun school, which is unusually late for a MD diagnosis.
The cause of your grandson's pain and discomfort is probably benign, but if it persists, I encourage your daughter to have the doctor delve deeper to find the cause. In the meantime, make sure he is eating a well balanced diet, with LOTS of fluids (even mild dehydration can cause a host of symptoms, muscle pain and cramping among them), ibuprophen to combat inflamation, benadryl combats inflamation too, and may help him to sleep. Try alternating a heating pad on a low setting with some cold packs. And have him do some GENTLE streatching exercises followed by massage when he goes to bed.
Hope this all works out for ya'll.
Peace,
S.