What would be the effect on a 9yo child's heath if he sleeps less than six hours in a day?

Efffects Everything! Sleep deprivation will effect a child's ability to learn, memory, behavior, coordination, weight and growth, immunity and the ability to fight off illnesses. Chronically tired kids don't focus or concentrate, are forgetful, are more likely to act out and have conduct problems in home and school, and are more prone to obesity, hypertension and diabetes as they age.
Miss his potential. A child who doesn't sleep enough will miss his potential and not make the best of his natural abilities. Some of these kids will seem fairly normal, while others will be tired, sleepy, irritable, forgetful, distractible, or hyperactive. Sleep deprivation can be from sleeping too few hours or from poor sleep quality (such as when a child snores). A pediatrician can help evaluate a poor sleeper.