My oldest grandson is currently 9-1/2. I'm trying to picture him in this child's position. They do not have a dog.
If they did, the only thing he would know about taking care of it would be what he saw his parents doing. He is not at 9-1/2 independently wealthy. He could not feed the dog on his own. He could not drive it to the vet, nor pay the vet bill. He would probably not even realize the dog was not healthy since he would have no prior experience with which to compare the dog's appearance.
It would seem that this boy really did LIKE his dog, and the dog apparently liked him. To me, that indicates that the boy was doing as well as he could with his pet given the circumstances. He was not neglecting, much less abusing this animal. The parents were. They may have been neglectful because they just didn't give a diddly, or they may simply have not known any better. But in no way could the boy be realistically blamed for the condition of the dog when the OP found it.