AAH Veterinary Clinic does animal boarding. In their ad, it says "small animal," so I don't know if this would work for your dog or not. The Vet. there, Elizabeth Nave, is an American. I haven't yet boarded my cats with her, but she has seen them in a veterinary capacity and I've been very happy with her. If they don't do boarding for dogs, I'm sure she would be able to tell you who to contact. Here is all of the info: AAH Veterinary Clinic, C/Pleyades 1, 28023 Aravaca. 91 357 4496. Fax: 91 307 1327.
Good Luck!