Yes I agree Alty.
For any breed of dog is is important for them to be with their mother until at least 8 weeks of age, it is in fact illegal in most states to sell a puppy any younger.
Great Dane puppies should be kept on the lean side while they are growing but from the sounds of it yes yours is underweight.
My first suggestion would be to get your pup wormed, worms can often cause weight loss and diarreah.
Next thing is what are you feeding?
Choosing the right food for a giant breed is extremely important. They need to be fed a diet of less than 24% protein to prevent deformities. I',m not sure if this is the case with your dogs back legs but it is a possibility.
Check out this site, the lady is a great dane owner and an animal nutritionist.
Great Dane Lady | Dog Health, Nutrition, HOD, Allergies