Type I diabetes, which is often called juvenile-onset diabetes, is not controllable by diet. It's an auto-immune disease in which the pancreas completely stops producing insulin. A child will not survive for long with no insulin, no matter what you feed him, because he will not be able to utilize any of the sugar/carbohydrate in the diet. All sugar remains in the bloodstream, and the kidneys struggle to reduce the load by creating more urine to dump the excess sugar (which increases thirst). This doesn't work for long. Meanwhile, none of the needed sugar is getting into the cells that are starving for energy.
Type II diabetes is a metabolic problem which usually sets in after insulin control has been disordered for many years, and can be aggravated or brought on earlier (even in early childhood) if the person has the genetic makeup putting them at risk, if too much sugar and other carbohydrates are part of the diet, and if the child does not exercise enough to make the muscles burn sugar. There is a possibility of minimizing symptoms through proper diet and exercise, but it's still a very serious disorder, and you can't deal with it without careful education and medical care.
With either form of diabetes, frequent blood testing is needed for good control. With Type I, control is achieved by injecting insulin, NOT through withholding sugar. With Type II, limiting sugar is important, but still not enough for proper control – you can't guess how high the sugar is based on how thirsty a child is.
If you think there's any chance your son might be at risk, please get him to a doctor immediately. Diabetes, if untreated, can cause tremendous damage in the body, to nerves, kidneys, blood vessels and heart, and other organs. And it can ultimately result in death.
(I've had Type II diabetes for 7 years, since I was 55, and taken care of myself with diet, excersize, medication, regular blood testing, and regular visits to my doctor for more thorough monitoring.)