high systolic in young athlete

I've got a 17year old son who competes in track & field.  He has always had a normal blood pressure.  At his last school physical they determined that his blood pressure was 150/61.  In the days subsequent his blood pressure has always been low diastolic, below 70; but the systolic never lower than 125.  He's also got a good pulse rate.  Takes no drugs or medications.

He's normal weight/healthy (6' 2", 164lbs); concerned because of the fact that his daily workouts are so intense.  What type of specialist would be best at diagnosing his problem?  Want to find someone who aggressively pursue the underlying problem.  I was told it could be nutritional given his heavy workouts (potassium,calcium)???