Given Duke's specific issues and restrictions and knowing you do not have experience in balancing a diet to meet his needs, I recommend you pay a canine nutritionist for a specific balanced diet to meet his needs. You can then learn more and read more to understand how to incorporate variety into it appropriately, but know that you are starting with a balanced diet.
Monica Segal (
www.monicasega l.com) offers consultations and her book provides guidance on balancing the diet and addressing the needs. You could also consider Catherine Lane (
http://www.thepossiblecanine.com/). Both Canadien but work mostly with the U.S. and are two of the most well known and published (see Catherine's article in this month's BARK magazine).
Of course, there are probably local options available as well and I know scores more who can provide this service, including myself, so don't be afraid to check around.
P.S. If every dog were given an allergy test, they would test allergic to soy, flax, and at least one starch metabolically. Those tests are often hard to go by as a result but still a good thing to avoid in food nonetheless.