Did the vet say anything about a Taurine deficiency? Most dog foods dont supplement Taurine (Amino Acid) because the dog can typically make it on their own from cystine and methionine....cats however cannot so their food is Taurine supplemented (deficiencies cause eye degeneration, possible myocardial problems and lethargy). It may be that he was doing well on the cat food because of that extra supplementation.

So for a dog that has no problem with Taurine that food might not be so bad, but it could be very dangerous for a dog that has problems synthesizing Taurine on his own.

-Ceph