Dear Karolin,
As was described above, left ventricular hypertrophy (LVH) is abnormal thickening of the heart muscle. However, it is important to put this in context of the rest of the history and examination, and why the ECG was obtained in the first place; the ECG result should not be taken by itself. Furthermore, ECGs are really good at assessing heart rhythm abnormalities; they are only okay at assessment of chamber size, such as LVH. In fact, LVH is frequently overcalled on ECGs. If there is true LVH, causes can be from highly trained exercise, from obstruction of the outflow of the left side of the heart, or from a disease called hypertrophic cardiomyopathy, a genetic abnormality of the heart muscle. If he is otherwise normal, the likelihood of true LVH is low.
Hypertrophy is thickening of the heart muscle. Is he really active in sports? Sometimes it can happen with aerobics training, but normally is indicative of heart disease.