Again, sounds like this dog has been let down. Just think of all the stress he has been through...the transportation, being sent to a new home, ending up at yet another pound. He needs someone to step up to the plate, spend some time with him, not to look for reasons to kill him. The expectations of a perfect dog are unrealistic at best. Who knows what kind of a home he came from? The stressful nature of a shelter or SPCA in itself is good reason to cause even the most normal dog to act out.
For a dog to be a shelter since January is such a long time. Shelters or SPCAs are not homes no matter how many volunteers they have. Is the shelter overcrowded or are they just tired of this dog? If he is now being deemed unadoptable, how come it took them this long to figure this out?