Your Rottie is still a pup, right? What kind of mineral supplements is he getting? The reason I'm bringing this up is when I was a teen, about 13 or 14, my legs would hurt at night. It wasn't because of sports or exercising. I think it was "growing pains". I started taking dolomite tablets, natural source of calcium and magnesium (we didn't have chelated minerals in the '70's) and the pain in my legs went away. If I forgot to take the dolomite, my legs started hurting again. They never gave me any problems during the day, only at night when I went to bed.
Maybe a good multi-mineral ascorbate powder would be good.
Maybe something like this: http://tinyurl.com/yccmlu8