My 12 yr old son says he feels a heartbeat near his stomach when he lies down.he says its been happening for four weeks. what should i do ?

Abdominal aorta: Junior is at the age where he's learning about his changing body, and he's almost certainly discovered his abdominal aorta. He's probably lean and in good aerobic shape. If you're not familiar with the pulse yourself, encourage him to see which of his friend can feel theirs. Many of the medical students are surprised to find it when we teach the physical exam.