It is good news they are helping him through diet. I have not used ZD or DD but have had a couple of cats on prescription Science Diet. One was Medical (Sam was diabetic) and the other is the Metabolic. One thing I have always added into my cats food is pumpkin. I buy a can of it and mix it in for a few days a month. I have seen it work with both dogs and cats. Some cat foods now are actually adding it in. You don't have to give them a lot but mix it in with their wet food. Please give Sylvester and big snuggle for me and let me know how he makes out.