Chest Palpitations, Sluggishness, Anxiety. Normal Thyroid. Suggestions?

My son has seen a heart specialist and his family physician. He is 16 years old and continues to have chest palpitations, feelings of sluggishness, and at times a little anxious. The heart physician after running many tests could not locate the problem and his phyisican ran a thyroid test which revealed a normal thyroid. I felt my son s muscle just about popping out of his chest under his left breast - it was quite disconcerting. His heart beats quickly during these episodes. The episodes occur when he is at rest, sitting in a deer stand, going to sleep, etc. He is very active in sports but does not seem to have these issues while working out or conditioning. Please offer any suggestions as I am at a lost to which avenue to pursue.