x2 my thoughts exactly
As for your girls, I agree is doesn't look too severe. I would recommend making sure their wounds are clean and then spraying Blu-Kote on them. If you can, order some hen aprons. That will help reduce pecking and also protect their bare backs. I ordered mine from Tess here on BYC. They were really good and I liked them, not to mention cute!
Granted, the elastic does wear out kinda quickly, but that can easily be replaced.
Other than that, make sure your dog isn't left alone with your chickens. If he's allowed to continue this it will be impossible to break the habit. Nip in the bud now so it doesn't happen again. He needs to be trained, and until he is he should not be with the flock unsupervised. It's not fair to him or your birds. He's trying to play and they are suffering for it. I have a dog who occasionally will try to play with a chicken and get too rough. He just has to be reminded that chickens are ABSOLUTELY NOT allowed near his mouth. I got him as an adult and his previous owners had allowed him to chase their cats, so he thought it was okay. I ultimately had to get a shock collar. I didn't use it for very long, and I didn't set it to very high, but for a little bit every time he would put his face near a chicken he would get a little zap. He learned very quickly that he was not allowed to chase them. I don't recommend a shock collar unless absoluetly necessary. If you overuse them they can do more harm than good. But it may be a good (very short) beginning for a dog already in the habit of chasing and tearing up chickens.