Hello and welcome here. A couple things determine his starting dose. First, take a look at the yellow starred Sticky Notes at the top of this forum. A lot of info in there, don't worry about getting it down the first read.

The Sticky Note on Dosing Methods has some ideas on what dose to start with. If he's never been on insulin before, you have one decision to make first, and that's whether you want to follow the Tight Regulation Protocol for dosing or the Start Low Go Slow Method. It's really up to you, your lifestyle and goals. Since Sullis is just eating wet food, you can choose either one. If you want to follow TR, we'll need to know his weight.
Can you tell me a little bit more about how Sullis was diagnosed?