[anonymously]

I would head back to the vet, tell him the treatment prescribed has not been successful, so far, and that the symptoms have returned.
The dog may need some additional tests and pharmaceutical intervention. In the meantime, I would keep him on the bland diet, whatever works.

PS: The dog may have picked up a nasty canine virus at the kennel. Sometimes they need sq fluids and anti-nausea medication to get them over the hump.