Our son went through growing pains, and still does occasionally (he's 12 now). It tends to be more noticeable if he has been really physically active.. as that does a lot of stretching of the muscles when they are already stretched due to growth spurts.
But, he also had a strange thing go on around age 5... we were on a trip, and when he woke up in the morning, he complained of his legs hurting. It hurt to extend them while standing. Just extending them will lying down wasn't a big deal, but he couldn't do it with any weight on them. So he hobbled and cried trying to walk. We thought, oh growing pains, etc... it'll relax as the day goes on. Well, it didn't really.. of course, we were in the car most of the day, so he didn't get to stretch much. But, he had been fighting off a cold, and that particular day didn't drink much of anything but water (pushing fluids due to the cold). The next morning, he was still unable to stand and bear the weight of his body, so we took him to the ER (we were at my mom's out of town and it was a Sunday, so couldn't see our regular doc). They did all sorts of bloodwork, and really found pretty much nothing. While waiting for the results of the blood work, he was thirsty/hungry. So I walked down to the vending machines and got him a Powerade and pack of crackers. He drank about 1/3 of the 20 oz Powerade, and when the Doc came back in to go over the blood work results, and recheck his legs again, he was FINE. COMPLETELY recovered from whatever it was. At this point, I believe it was more of an electrolyte imbalance. His bloodwork showed they were slightly off, but nothing dramatic, but then they didn't really have "normal" values for a kid his size (if i remember what they told me correctly) without having run the bloodwork when he was healthy as a benchmark. So maybe it was really WAY off for him.
Obviously, I wouldn't not go to the doctor based on what I have shared about our son's story, but it certainly couldn't hurt him at all to drink an electrolyte beverage or two to make sure everything is in balance.
Just a thought..