What could be casuing a epigastric pulse in a 15/yo boy , who doesn't smoke, drink, no family history of aaa. epigastric area pulse, when laying down.

May be normal: In thin people you can often feel the pulse of their aorta when palpating the abdomen. This can be evaluated by your pediatrician or any physician.

AAA uncommon: AAA would be very uncommon in a normal healthy 15 y/o. Some rare conditions can cause aneurysm in young people, for example even young people can develop mycotic aneurysm (due to infection). Or genetic diseases like some forms of ehlers–danlos syndrome (eds) can cause aneurysms.

Likely normal: In a thin male, the aorta is likely to be palpable and may even be visible just by watching his stomach. An aneurysm at his age would be disgustingly uncommon.