Hallo
Could you tell me what kennel you brought him from ? were his parents hip scored? usually even on a complete diet Dane breeders give a diet sheet the dogs are usually fed a food for giant breeds along with meat, rice pudding all that sort of food they have to have good levels of calcium as the bones are so large.
It is certainly not good news to have a young dog or in the case of a dane mid aged dog with joint problems.
As for the behaviour I agree with Kiz reading your posts it does sound like you are encouraging his behaviour, be it that you do not know it
Val