You can feed them when their numbers are high. It usually won't elevate them too much if you're feeding low carb, wet food. You actually want to make sure they're eating when their numbers are high because a lack of appetite + not enough insulin are the first two ingredients in a recipe for diabetic ketoacidosis (the third being an infection of some kind).
Is he throwing up because he's eating too fast? Maybe has some tummy issues (licking lips, walking up to food and then walking away)? How are his 5Ps (Preening, Pooping, Peeing, Playing, Purring)?