Thank you for your query on Healthcare Magic.
Racing heart is usually is associate with paleness due to scary things which will activate sympathetic system of the body leading to increased heart rate
It is due to release of adrenaline
from adrenal gland as a reaction to external stimuli which is know as fright or flight reaction.
It is a normal reflex and is seen in all people. But if he is getting them frequently he needs evaluation.
But any underlying heart rhythm abnormalities also to be ruled out by doing EKG, X-ray chest, and 2D Echo.
Therefore I advise you to consult your Pediatrician
for the examination of his cardiovascular system to rule out any rhythm related problems by appropriate investigations.
Hope I have answered your query.