Lilysam

Hi everyone,
So my almost 4 year old GSD has been having some intense digestion issues for the past year or so. We have him on a prescription Royal Cannin gastrointestinal low fat kibble diet. He is digesting it well, with an occasional flare up here and there, but it is causing him to loose weight. My once 95 lb dog is now down to 80 lbs. He is very active, and I am worrying about him not getting enough nutrition for his lifestyle, the ingredients also aren't the type that I would want him to be eating. However, whenever I try and switch him gradually to another food that has higher protein and better, more whole ingredients, it upsets his stomach and he has a hard time digesting.

Is there any supplements he should be taking to help give him what this food is lacking, or better yet is there a food that is better than what I am using and can still be easily digestible?

Thank you for your help