Hi, you are doing the right thing for him! I had the same issues with my cat (who had kidney probs), and one of my dogs. My cat seemed to constantly be pestering for food when he first went on the prescription diet.
The dog was overweight and she needed a special low fat diet. For a couple of months she must have been starving on the amount of food we were giving her compared to what she was used to. I felt terribly guilty. But a year down the line, she is far healthier and happier than I have ever seen her.
Guess you just have to keep in mind that ultimately you're doing the best thing for your pet. Stick with it! Good luck!