Im sorry that your boy is having this problem. But I do want to suggest trying something different than Science Diet. Now this is all just IMO but it is not a good food, prescription or not. Most vets push SD because that's what they're taught is a good food and its the only one dogs with medical issues should eat. Also, the change to SD might not do well for him because of the quality difference between what you feed now to the (IMO) crap that's in all SD foods
(Again just IMO) I see switching a dog from a good food like Core as saying "okay I eat 3 square and balanced meals a day, but now I'm gonna eat only mcdonalds and greasy pizza because the doctor said its what I need"
There are many foods out there that are high quality and low protien. I just looked at the ingridents for the prescription food. That's just gross. There's nothing healthy about the top five and those are the main body of the food.
Okay I'm really sorry I rambled, in the end its all up to you, I just try to help people know that there are better options than Science Diet. You can also google canine nutritionist (sp?) and consult with them, also talk with your Chinese vet too about feeding options