My 5 yr old son's feet are always cold and this morning he woke up saying they were pounding like a heart does. What can it be im really scared?

See a pediatrician. This could represent a benign vascular tumor or a neurological problem. See your pediatrician. If they can't give you a diagnosis and treatment a referral to a vascular surgeon or neurologist may be warranted.
Growing pains. It may be nothing. Kids at 5 sometimes complain of pounding pain in their feet due to fatigue from flexible flat feet. That being said, it is important to rule out a vascular problem so visit your favorite podiatrist for an exam. If it is growing pains then stretching, better shoes and orthotics can help.