Short QT interval in oligosymptomatic teenager boy

He is a boy who just turned 12 years old these days. He frequently plays soccer, and also rides bicycle. Asymptomatic until a few weeks ago, when his father noted that «does not perform» physically (??) as before. When questioned, the child answered that he «gets tired» when he plays (??). Doubtful presence of palpitations, and of some sporadic dizziness during the effort.
Without family history of sudden death, syncope, or other heart diseases.

Physical examination: Blood pressure: 105/70 mmHg; pulses all present, heart rate about 72-78 bpm, with sinus phasic respiratory arrhythmia. Auscultation: Normophonetic first and second heart sounds (S1-S2), systolic murmur barely audible, 1/6 in mesocardium and pulmonary focus suggestive of innocent murmur in nature.
ECG (see slides 3-5): Sinus rhythm, normal PR interval duration, short QT interval (II 350ms in II and 320 ms, in V6 early repolarization pattern in anterior wall.

Echocardiogram: normal.
Holter (see slides 7 and 8): I am struck by the short QT interval and the image «in spoon» at the beginning of the ST segment, with persistent depression. Only 2 isolated supraventricular premature contraction were recorded.
He complained of dyspnea coinciding with the efforts, sinus tachycardia and nothing else.