Ketones are produced when the body burns fat for energy or fuel. This can happen when you lose weight or if you are unwell and have not been eating properly. Also if there is not enough insulin to help your body use sugar for energy. Without enough insulin, glucose builds up in the blood. Since the body is unable to use glucose for energy, it breaks down fat instead. When this occurs, ketones form in the blood and spill into the urine. These ketones can make you very sick.
There are situations when you might have ketones without the blood glucose being too high. Positive ketones are not a problem when blood glucose levels are within range, It is a problem if blood glucose levels are high and left untreated.
I would suggest just checking with your DSN tomorrow and in the meantime carry on checking your sons levels through the night and correcting if necessary. Also get him to drink water as this will help flush the ketones away.