The poor dog....

I'm pretty sure though it's implied that he was being fed by his owner who while being a scumbag who faked his own death and stalked his ex wife, did care enough about his dog to feed and take care of him. If you remember it's 2 weeks and a few days between the time of the beginning of the film to the point she goes back to that very place. If nobody fed him and watered him then he'd be dead when she goes back there. Even a dog will die of starvation or dehydration well after a few days just like a human being. So he had to have been taken care of those 2 weeks and a few days or else he'd be dead. Or at the very least have run away to find someone who would take care of him. A dog doesn't just allow itself to starve like that without running off to find someone who might take care of it.