hmmmmm a Beagle that doesn't like his food is a very strange issue to me. Have you had him checked out with the vet. I feed my Beagle IAMS (I know many people out here don't like IAMS, but I had him on Science Diet and he didn't do well on it.) I am thinking about changing him to eucanuba with the next bag.
But, back to your question, are there any other symptoms which are different about him, how old is he?