Welcome!
What insulin are you using?
Dry DM is high in carbs so that is not ideal to feed him. A lot of us feed Friskies pate or Fancy feast classic pate, none of the gravy types. I feed my cat 9Lives. There is a list of food done by Dr. Lisa that lists the carb count of a lot of wet foods. You don't have to feed the expensive food from the vet- there is nothing in them that actually make them "prescription" food.
Dry food is not recommended. BUT there are two that are acceptable carb counts: Young Again Zero
www.youngagainpetfood.com and EVO cat and kitten (purple bag I believe).
You can feed more than twice a day. A lot of us on here feed multiple small meals a day.
It's best to test at home. You can use a human meter. I use ReliOn Prime. Meter was about $9 and the strips are $18 for 100. There are other relion meters that use a smaller sample of blood. There are videos on here on how to test your cats blood glucose that can show you how. Treats are a great way to help convince him it's a positive thing.
Edited to add: It's very important to monitor his blood glucose levels and adjust the insulin if you decide to take him off the Dry food and switch him to a low carb wet food from the DM wet and dry. It must be done slowly to make sure he doesn't drop too much and end up going hypoglycemic. Cutting out dry food can drop their numbers by hundreds of points.