Are the gums and tongue red? Panting when not hot or stress, like to going to visit the vet (they stress and pant), panting is a bad sign!!
Please call another vet asap, explain over phone your issue, see what they say!!
Some vets are human, they can make a mistake, or miss it, or just incompetent,
but, not eating, it could be internal damage, got to get xray, just insist on xray, your dog is in trouble!!!
Welcome to the forum, there are wonderful great people here...I am not trying to sound morbid to scare you or hasty, I beleive you have a dog in trouble, dogs dehydrate quick, and too much pain in a dog will stress the heart and the heart can enlarge...it can only go downhill quick...
Please try to contact the opinion of another vet!! Let us know, keep posting, so very sorry about your little friend distress w/the accident, hope you get the help you need!
Is your dog's pain being treated? If so, with what meds? Was he checked for internal bleeding? What really bothers me the most is that he isn't drinking enough, meaning dehydration is a real danger. He may need to be put on an IV to bring his fluid and electrolyte levels back in balance. When did the accident happen, and how long has he been like this?