I am very concerned for the health of your chow chow. I would suggest doing an xray to rule out something that may not have been picked up by the sonogram. I did see a case of a 10 year lhasa apso who ingested a piece of something which appeared to look a paper clip on xray. It was lodged in her stomach and she did exhibit chronic vomiting. A sonogram was done as well, and there was no sign of this object found. The xray was the only thing that showed this. Xrays with a barium contrast (if he is able to hold down the barium) would be helpful in diagnosing a decrease in motility, megaesophagus, and foreign bodies. If he can't hold down barium, a plain old xray could be helpful as I mentioned earlier.
Since this has been going on for so long now and your pet is losing a significant amount of weight, you may want to consider hospitalization.
It is possible that your dog also has inflammatory bowel disease. This can cause vomiting, and/or diarrhea depending on which portion of the digestive tract if affects. It is very difficult to determine the cause of IBD, but, it will sometimes respond to cortisone, coating agents, and antibiotics.
Assuming that xrays are negative, there are other medications which can help your pet. Reglan is used to help empty the stomach faster to decrease vomiting. Another is carafate which is a coating agent. Cyproheptadine can be used as an appetite stimulant but can cause your pet to become drowsy.
If you can, seriousl think about hospitalizing your pet. Your vet can give injectable medications instead of or in addition to oral ones. He can also be put on intravenous fluids to'prevent dehydration. If you are not happy with how your vet is treating or diagnosing your pet, a second opinion would be helpful.
In the meantime, I would offer your pet some Ensure which you can find at most grocery stores or a pharmacy. Get the vanilla flavor. Most pets like it. It will add extra calories to his diet as well as vitamins. You can also try stage 1 baby foods such as chicken or beef in broth.
Don't delay in getting him the medical attention he needs. He is an old man, and I would hate to see his condition worsen. Best of luck to you and let me know if I can assist you further.