My son's heart while at rest or while sleeping beats at a normal rate but fluctuates in speed ever few seconds. kind of like a car's idle that surges.

Chances are it's a : normal sinus arrhythmia, but his pediatrician needs to know his age, resting heart rate & other symptoms to perform a physical exam & determine if a pediatric cardiology consult is needed + 24-hour monitoring of his heart rate & rhythm. See http://www.heart.org/HEARTORG/Conditions/Arrhythmia/AboutArrhythmia/Types-of-Arrhythmia-in-Children_UCM_302023_Article.jsp#mainContent