I would think that you should add some more meat into his diet, sardines are obviously fish which is used alot for its low fat content. Also got some cash to burn hey morgan
not too many I know are keen to feed a dog 6 cups of advance a day :P.
I cant remember the exact specs on the advance food but I imagine to be moderate fat content not that high, the weight gain you see in this instance will be pretty much proportional to the fat in his diet. Does it say on the advance bag how much is rated to give a dog his size?
Another thing is if you have the contact number of the person you got the dog on, get an idea of what they were feeding him as they were obviously able to keep his weight where it is, if you use this as a benchmark you can add from there.