Yes, I think that you could.  If they can prove that he was their dog and that he was just lost then a court could order the dog be returned.   I know that it may indeed be a difficult thing to come to terms with and that you have grown attached to the dog.  I think, though, that they may feel the same way.  If you think that the dog was abandoned or abused then make a case for not returning it to them.  Good luck.