My son has mitral valve disease. Should I be concerned?

Depends. Depends entirely on type and severity. Generally there are 2 kinds of problems:blockage (aka stenosis) (almost always from rheumatic fever) and leakage (regurgitation). Mild forms require no treatment and cause no symptoms. Severe forms are correctable. You need the details before deciding if there's any cause for concern.
It depends. It depends on the severity and the underlying cause. An echocardiogram can beextremely helpful .