Welcome to FDMB.
It sounds like you're aware that your cat needs to be eating differently. I would not make a drastic change to his food since I suspect what he's eating is high in carbohydrates and may be off-setting the amount of Vetsulin. Most of the prescription foods are high is carbohydrates, many cats end up not liking them over time, and the ingredients are such that you're not getting what you're paying for. The other issue with prescription foods is that there is nothing in them that requires a prescription. The pet food manufacturers lost a class action suit for calling their pet foods "prescription". For the cost of the Hill's food, you could be getting a much higher quality food. You also have a lot of options for low carb foods.
This is a
chart that contains nutritional information on most of the canned foods available in the US. We consider low carb to be under 10% carbs. However, most members feed their cats a diet that is in the neighborhood of 5%. There are only a limited number of dry foods that are low carb -- Young Again Zero Carb and Dr. Elseys Clean Protein. ZiwiPeak has an air dried raw formula that is low in carbs, as well.
If you're not already doing so, I would also encourage you to start home testing. If you are planning on changing to a lower carb diet, with the dose of Vetsulin, there's a very good chance your Buddy's blood glucose numbers will drop. The only way to be sure he's in safe numbers and not becoming hypoglycemic is to home test. Many of the members here use the Walmart Relion meter to home test. The test strips for the Relion meter are among the least expensive available. The cost for strips for any human meter will be far less than the cost of strips for a pet meter. We are huge advocates of home testing. Many vets overlook recommending home testing or they have had little luck in convincing their clients to home test. That said, the veterinary professional organizations encourage pet owners to home test.
The other concern that I have (and I'm sorry to be voicing all of these issues) is the insulin Buddy's been prescribed. The American Animal Hospital Association stopped recommending Vetsulin for the treatment of feline diabetes in 2018. Its duration of action is far too short for a cat's fast metabolism. They recommend either Prozinc or Lantus (which is now available as a generic/biosimilar -- glargine). In the meantime, I'm providing the link to our forum that has
information on Vetsulin. You may find the sticky notes at the top of the board helpful.
Please let us know if you have questions. The members here are very generous with their time and knowledge. We're here to help.