I know this will sound odd, but they have preferences just like we do. I had a dog what wouldn't eat or drink out of ceramic or metal bowls, but would out of plastic. My current gang seems to prefer the ceramic, and out of the 2 ceramic bowls we use, for awhile wouldn't eat out of the blue one, only the tan or yellow ones. They now will eat out of all 3.
Had a horse once that wouldn't drink water out of black buckets, had to be either blue or red.
Try different materials, different colors, or even the plate idea that many here say they use. Just keep trying until you find something that works for your guy and mark it down as one of his special little quirks.
