Shawna,
The dry food you are using is one of the lowest carb dry foods available. It is listed on Binky's list at 7% carbs, which is okay from a carb standpoint for a diabetic cat. There are other reasons why long term you may want to remove it from the menu, but for now, it is okay as long as he isn't eating a ton of it. The most important thing is that he will eat it, because when you are using insulin, eating is very important.
And yes, I know this can be scary, terrifying, overwhelming, all of that. It won't be that way forever. And the more you post and ask questions, the more you'll realize that you are not alone in this. We've all been in the exact same place you are, and we all want to help.
Carl