A vitamin deficiency should be easy to fix with the right foods! What food did your vet recommend? Zupreem pellets are really good, but remember that birds need other foods along with pellets, such as veggies, some seeds, etc. Hope he'll get all better soon! I'm sure he'll get better once he's on the right diet. Cockatiel Exact Rainbow Pellets probably weren't right for him because he's not a cockatiel, and every species has their own nutritional requirements!