hardug

My son is 27 yrs old. He is physically fit and healthy but 6 months back about Feb/Mar 2018 all of sudden ,he complained of fast heart beat rate.I dismissed his words by saying may be due to stress and all.But again he complained of breathlessness and fast heart beat.I took him to nursing home and doc tested him by ecg,bp etc.and everything was ok.Doc told me that these things are arising now a days in young generation due to pressure of work and irregular life style. Then he went abroad and again after few days he had this problem.I took his case to one of the famous cardiologist and she advised to go for holter test.According to her,there may be some extra electric circuit in heart and due to which it happens but will be confirmed by holter test.I sent him 1 mobile holter device by which one can get immediate ecg readings and measure heart beats.Whenever he experienced the above said symptoms,he took the readings and mailed to doc and every time according to doc it was perfect.My son also went for proper holter test in the hospital and readings were absolutely fine. But still regularly he gets these symptoms once in two months or so and we are unable to diagnose why it is happening. Please advise what should I do to diagnose the root cause and then go for a proper treatment. Please note that in every other respect he is absolutely fine and leads a normal life like any young boy. But when this problem occurs then during that short time period he feels weak ,nervous and difficulty in breathing. I am at a total loss to move further as even the best of cardiologists aren’t able to advise anything , or may be this is not even a cardio problem that’s why need someone to guide properly for next step in investigation towards a proper diagnosis of the problem Doctor has asked him to take medicine Metolar 25 mg and Aciloc for 3 months only which he took and now whenever this problem happens he drinks lot of water and Aciloc, this breathlessness lasts for 5-10 minutes and then becomes ok on its own but after each such episode his energy is totally drained out and he feels tooo weak that’s why guidance needed to which specific doctor he should go for proper check up and diagnosis