If a child is on the ketogenic diet, what is he eating?

Fats and proteins. A ketogenic diet is very low in sugars, almost none. The idea is the ketones from digestion without sugars may lessen seizures. The diet is quite difficult and restrictive. It is typically only used in severe cases where multiple medications are not controlling the seizures. A nutritionist is always involved (along with the neurologist), to ensure that all vitamins and minerals are included.
Protein and fats. The ketogenic diet, usually used for severe difficult to treat seizures, is a diet with low or no carbohydrates, similar to the atkins diet.