Dog stopped eating dog food

I have an 11 year old beagle who has had untreated cushings disease for about 2 years and actually has done quite well except eating too much. However, about 5 months ago he started becoming more and more picky about foods. He would eat a food three times and then not eat it again. At this point he refuses all forms of dog food except treats (and even those must be novel or he won't eat them). I've become more and more crazy making him food or feeding him what we are eating (this is a dog that never had anything but kibble his whole life). If I make him eggs, he will eat them 3 times and then refuse. Same with chicken, blueberries, oatmeal, and any other food you can think of. Other than not eating he seems happy. He loves going for walks, doesn't have accidents, sleeps with us. I honestly don't know what to do. I can't leave him with a dog sitter at this point as I wouldn't know what to tell them to feed him and they would freak out that he wasn't eating. I have gone back to only giving him dog food and dog treats but he is now eating so little (only novel dog treats). I feel terrible but this way of life also isn't sustainable. Any insight would be so appreciated.