Hi, welcome to FDMB! You have come to the right place for answers!
I take it your kitty is not diabetic, but you and your vet are concerned that he may be headed that way? Your vet may have accidentally steered you in the wrong direction regarding food. For our diabetic kitties we recommend feeding a diet that is very low in carbs, under 10%, and preferably canned/wet food. Almost all dry food is
very high in carbs, and helps lead to dehydration, as well. There are only two dry foods that are low-carb enough for diabetic cats - Evo Cat and Kitten and Young Again Zero Carb. I have heard a rumor that the Evo is being discontinued, don't know whether or not that is actually the case. The YAZC is expensive but very calorie-dense; your kitty will only eat a very small amount per feeding. If you contact the company they will be happy to send you a sample. (My cats loved it, it was like kitty-crack to them, lol!)
There is absolutely no need to feed expensive "prescription" food, there is actually nothing prescription about it and several of the companies that make it are now facing class-action lawsuits because of it. I know your vet wants the best for your kitty and thinks the "prescription" food is the way to go; most vets get very little training in vet school about feline nutrition and feline diabetes, and as a rule not too many vets actually have dealt with many feline diabetes patients. The salespeople for the food contact the vet and tell them how wonderful this "prescription" food is for cats with FD and the vet figures it must be, or they wouldn't be pushing it, so don't be too hard on your vet, they really think they are doing what's best.
Here is a link to a great article, written by a vet, regarding feline nutrition:
www.catinfo.org
and a link a food chart showing the breakdown of protein, fat and carbs in many readily-available cat foods:
http://catinfo.org/docs/FoodChartPublic9-22-12.pdf
Hope this helps!

