If it is a small amount (one level teaspoon) once, maybe twice a day, then it should not adversely affect his blood glucose. However, tuna is almost all protein and very little fat. Excess protein in relationship to low fat will cause the blood glucose of a cat to spike. Remember, your cat is a carnivore and can easily convert excess dietary protein into excess blood glucose.
Since the tuna is actually a treat, you are actually encouraging your cat to eat when they may not need to. Overeating will also cause an unnecessary increase in your cat’s blood glucose. Best to keep treats to a minimum with diabetic cats.
Play time or brushing can substitute for treats and your cat will love the extra attention.
A better choice for a treat would be a canned food that does not contain any plant ingredients (animal protein/fat only) and has a balance between protein and fat. Two parts protein to one part fat would be the best option for your cat. There are very few canned foods available that meet these requirements and you would be best to only give a small spoonful at a time. The treat can be very helpful in getting a kitty to cooperate with giving a blood sample and is often used for this reason.