My son got this too. he also had it in ankles and heels. We had a orthapedic check him out - said same thing - growing pains. Hot bath before bed, advil and can rub his legs when it is really bad. Also a heating pad helped.
I to use to get this and I really think he is dealing with "growing pains". I remember these use to be pretty bad and upsetting but they will pass. I got them right in the areas you described horrible pain in the calves.
I think they will pass but I am no doctor mine kind of just went away. Growing pains can hurt pretty bad.
I was just talking about this with my Mum the other day... when I was little I used to get this very same thing. I know it's annoying when people say 'it's growing pains' but I do believe this phenomenon exists (whether or not it's actually caused by growing!). I remember it being always at night and always in my legs, a crampy, deep muscular pain that no-one could explain. It just went away, probably when I was just a little older than your son. So hopefully he won't get it too much longer! If he's been tested and they can't find anything, I wouldn't worry too much and just give him some kids paracetamol when he gets the pain.
I hope it goes soon, I do remember it being horrid.