At least....until he's better controlled, that may not be enough calories for him since he's diabetic.
Each cell in the body requires glucose to live. Think of insulin like it's a "key" and on every cell of the body there's a "lock"...if you don't have the right number of keys, the cells starve to death because they stay locked.
That's why diabetic cats can be eating huge amounts of food and still be losing weight. No matter how much they eat, the glucose just stays in the bloodstream (thus the high BG numbers)
Now that doesn't necessarily mean you should let him eat as much as he wants, but he may need more than a "normal" cat until he's better regulated.
Here's a good post to read titled
"Feed kitty as much as they want?"