This depends on the stage of pregnancy. If first trimester, you may have had a miscarriage. If second or third trimester, the baby's heart to far away from the surface to hear. If past 4 months, tap on your stomach to get the baby to shift.
He can check at a later date to see if you can hear it, but if he cannot then he may have to do an ultra sound to see if the baby is gone. There is a good chance it's still alive but you were unable to hear the heartbeat.
This seems to be a rare case. If he is nourishing well and don't have in connection with this problem there may be nothing nuch to worry. However this may be due to minor heart problem. Watch his activities carefully and consult a good Cardiologist dealing paediatric cases.