If you do find the dog, I'd take him straight to the vet. I don't know much about rehabilitating a starving dog, but, I do know that you have to slowly feed them and get them back to a healthy weight. Not just let them eat a lot of food right away. There should be a record of your call to the SPCA with the information about the dog, so, if the vet were to think you were the one starving the dog, you should be able to prove that he isn't yours. I don't know, I would take him to the vet, regardless. I hope you can find him and help him out.