Need advice about my nine month old puppy. on feb 8 he had surgery for legg calve perthes disease?

about 8 days later he became very lethargic, vomited once, will not eat ,drinks very little and has lost weight. also his teeth have turned yellow the vet could not remove the yellow so did lab work feb 19. no billirubin in urine...only labs out of the ordinary were ALT slightly elevated. he got IV fluids / lasix. was sent home on daily lasix and denosyl and returns for more lab work on feb 22. his physical condition has not changed. he eats very little with much coaxing and i give pedialite with a syringe in his mouth. what should i ask the vet? or should i go to another?