There are other manufacturers of veterinary prescription hypoallergenic diets such as the Hills Science Diet prescription and Purina Veterinary prescription lines. I am not certain that this is what your boy needs. He did eat when you added canned dog food for a few days. Please consider giving him something as simple as a raw hamburger patty, chicken breast, neck, drumstick, steak or something to see if he wants to eat. If he does not want to eat, you have an entirely different issue at hand.
I do know first hand how maddening this is because Max just went through this with his inflammatory bowel disease. He now eats every meal as long as I put something raw like a chicken neck, turkey neck, chicken back, drumstick, hamburger, chicken breast, etc. in the bowl with his kibble. He eats it all, so I know that he is interested in eating. It just took this type of enticement to get him to eat consistently.
It is frustrating to have to go through this, however, when they begin to eat consistently, it makes your efforts so rewarding and justified. I hope that this helps.
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge