I'm going to come in on the side of Cheryl and say it may not be such a good idea to carb-reduce in a teenager. Keep in mind, they are growing and experiencing brain development. For that, they need glucose. LOTS of glucose. And to process the glucose, they need insulin.
Understand that ketones, per se, are not evil. They are the product of fat burning and are a good and needful thing when someone exercises or is ill. Insulin is the natural limiter of ketone production, because in someone with a functioning pancreas, the presence of insulin signals that there's glucose in the system to be processed, and so fat burning stops. Ketoacidosis comes about in diabetics when there is insufficient insulin to limit ketogenesis to what the body needs — that is to say, in a person whose pancreas has shut down but hasn't started insulin therapy (many T1s get diagnosed because of DKA) OR in a T1 when there isn't enough insulin going in to trip the "don't burn fat" switch.
Now, my endo has said that if an ADULT diabetic were to never eat carbs, then all they'd need is a very small amount of basal insulin to prevent ketogenesis, and that would be it. BG would stay stable and they'd never need to inject boluses. But in a kid, that's not realistic, because growth hormones and other factors screw up insulin transport — makes teens insulin resistant, in short. So teens actually tend to need more insulin than adults overall, as well as more food. I have never known any teenage boy who didn't want to constantly stuff his face, and I think if you attempt to limit what he eats to certain foods or certain types of food, you're going to wind up with a sneak-eater, which will make his BG less stable. Counterproductive.
Then there's the whole issue of the fact that teens are developmentally attempting to establish a separate identity from parents. Think for a minute how that works with you attempting to reduce his carb intake in the interests of stable blood sugar. It may start out well... but having raised two teens already, I can promise it won't end well. He'll rebel. NOBODY likes to be told what to eat, and that goes triple for teens, probably quadruple for diabetic teens. And, in reality, he does not need a special diet — that came directly from my endo, when I asked the same thing. He can eat anything he wants (within reason — junk food isn't good for anyone) and stay relatively healthy, as long as he boluses for the carbs.
Bottom line, I think you should let your son eat whatever he wants and dose for the carb count he eats. You will have enough difficulty instilling the testing & bolusing habits without going into "eat this, not that" with him. Pick your battles, and make them few in number.