When first diagnosed and until he gets a little better regulated, it's really not important how much you feed. Our diabetic kitties are actually starving because no matter how much they eat, without insulin, the glucose can't get into the cells. Think of insulin as the "key" that unlocks the door to the cells to let the glucose in. There's plenty of glucose in the bloodstream (as you can tell by measuring the blood glucose!) but it can't get into the cells without that "key".
The "general" starting formula for feeding is [13.6 X optimal lean body weight in pounds] + 70 calories but one of the best things you can do is weigh Brody weekly...If he needs to gain, he should gain steadily until he's at his "optimum weight" and then you back down on the calories to keep him there