If the change was your preference and not due to a digestive or allergy-type issue with the dog, I would switch back to the Grasslands that you know he was doing well on before the switch.
As an aside, is there a difference in fat content between the two foods? Sometimes, changing from a venison/bison/beef kibble to one that is fish-based has a significant change in fat and that can produce diarrhea too.
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