The Iams dry food is high in carbs. Getting him off of that could lower his glucose considerably and help it get under control. So, it is interesting that your vet doesn't want you to switch until it is under control. Also, when you do switch, it will be super important to be monitoring his glucose, because it is very possible he will start needing less insulin. If your cat has lost weight due to diabetes, he should begin to gain weight once his glucose is under control.
Here are the ingredients in each of the foods. If you want, you can review them and decide what you are most comfortable with. When I was researching all of this, I learned more than I ever cared to know about ingredients I had never heard of.

Both of these foods (as is generally the case with dry foods) are high calorie. YA is 654 calories per cup and Dr. E is 554.
Young Again Zero Mature:
Hydrolyzed Pork, Chicken Meal, Chicken Fat, Chicken Liver Flavor, Guar Gum,DL Methionine, L-Lysine, Fish Oil, Psyllium Husk (source of soluble dietary fiber), Potassium Citrate, Brewers Yeast, Fructooligosaccharides (FOS), Potassium Chloride, Choline Chloride, Taurine, Potassium Carbonate, Vitamin E Supplement, L-Tryptophan, L-ascorbyl-2polyphosphate (source of stabilized vitamin C), L-Carnitine, Betaine Anhydrous, Magnesium Sulfate, Calcium Carbonate, Salt, Biotin (vitamin B7), Zinc Sulfate, Yucca Schidigera Extract, Rosemary Extract, Vitamin B12 Supplement, Niacinamide (vitamin B3), Thiamine Mononitrate (vitamin B1), Copper Proteinate, Folic Acid (vitamin B9), Vitamin A Acetate, Riboflavin (vitamin B2), Calcium Pantothenate (vitamin B5), Dried Aspergillus oryzae Fermentation Extract, Dehydrated Pediococcus Acidilactici Fermentation Product, Beta Carotene, Pyridoxine Hydrochloride (vitamin B6), Cobalt Sulfate, Vitamin D3 Supplement, Mixed Tocopherols and Citric Acid (Preservatives).
Dr. Elsey's Clean Protein Chicken:
Chicken, Dried Egg Product, Pork Protein Isolate, Gelatin, Chicken Fat (Preserved With Mixed Tocopherols), Flaxseed, Natural Flavor, Salmon Oil, Potassium Citrate, Calcium Carbonate, Fructooligosaccharide, Calcium Carbonate, Choline Chloride, Vitamins (Vitamin E Supplement, Niacin Supplement, D-Calcium Pantothenate, Vitamin A Acetate, Thiamine Mononitrate, Pyridoxine Hydrochloride, Riboflavin Supplement, Vitamin D3 Supplement, Biotin, Vitamin B12 Supplement, Folic Acid), Minerals (Ferrous Sulfate, Zinc Oxide, Calcium Carbonate, Manganous Oxide, Copper Sulfate, Iron Amino Acid Chelate, Manganese Amino Acid Chelate, Zinc Amino Acid Chelate, Copper Amino Acid Chelate, Sodium Selenite, Cobalt Carbonate, Ethylenediamine Dihydroiodide), Potassium Chloride, Mixed Tocopherols (Preservative), Taurine, Salt, Rosemary Extract.