Advice for When a Dog Won’t Eat, but is Otherwise Healthy

I have a six-year-old mini-cockapoo who regularly starves himself. I've tried multiple feeding methods, protein types, textures, and even mixing treats into his food, but nothing entices him. I've taken him to the vet about this multiple times, and they tell me that he's in good health for his age, other than being a bit skinny. Does anybody have advice on how to get him to eat on a consistent basis? This has been a problem ever since I adopted him at one-year-old, and it contributes to him constantly being anxious and angry. I want my boy to be happy but don't know what to do.