My other cat did not show any side effects then, but 3 weeks afterwards, he has gone off his pebbles, and losing weight, we took him to the

My other cat did not show any side effects then, but 3 weeks afterwards, he has gone off his pebbles, and losing weight, we took him to the vet yesterday they did tests, and said that he is stressed, gave him calming meds, he still does not want to eat, he only has a bit of Purr Puree, but not his pebbles, he drinks a lot of water, what else can I do. also want to say that we think he is stressed because of the new cat that has just taken over everything, or is it the revolution?