It is not uncommon for an indiviudal to have leaky valves. Mild, trivial, trace are considered medically insignificant and often do not progress, nor require any treatment.
If and when valve leakage becomes severe it can/will cause shortness of breath with mild exertion, chest pains, fatigue, etc. The sympoms occur as the heart does not meet the system's demand for adequate oxgenated blood. Some of the blood is lost to output with each heartbeat as there is backward flow into the atrium due to the leakage.
I doubt if your son would be medicated as treatment at his young age. If the condition is serious, the valve would be repaired or replaced. If it is not serious, the doctor would monitor for any progression.
Atenolol is a beta blocker. It blocks receptors to lower blood pressure. Does he have high blood pressure? If he is only 11 years old, his heart valve could get better on its own. Heart valves can improve, especially if he is still growing. Beta blockers have not been shown to increase lifespan. Beta blockers are good for lowering blood pressure to help take the pressure off of the heart.