How often is he worked/walked? Often the legs are victim to a dogs boredom licking. If he is getting plently of exercise and you don't notice him licking the area, a trip to the vet may be in order.
As for his fur turing a rust colouration; what are you feeding your rottie? Iams?
Most often, the fur turing a rust colour is because of a nutritional deficiency and Iams is notorious for turing black dogs, brown. It could also be hypothyroidism, or it could simply be because he is in the sun often...
Speak with your vet just in case.