Hi Silentandscared,
I would take him to a pediatrician if these fears don't subside. Has he watched any scary shows or played any violent video games. It could be that he's confused about death or is just starting to question death. When he's going through this, try asking him what has caused him to feel this way. Ask him if he has any questions about death itself. I know my own child was quite scared when she was 4 and her Grandfather died. Try not to show that you are upset when he goes through these episodes. If it doesn't subside then take him to the doctor.