It is natural for young children to have fears about death and dying because it is something completely unknown to them. If you are religious, then you can simply explain to him from a religious perspective what happens when people die. If you are not religious, you can simply talk about how plants die and animals die. Then briefly talk about how people die but usually not for a really long time when they are old. Tell him that we are here on Earth to be happy and that you and your spouse will do everything to help your son stay happy and remain safe.
At night when it is quiet, your son's mind is filled with many ideas that swirl around in his head which cause many chlildren to be afraid and anxious. If he exhibits such behavior in the evening, try to make sure that you establish a calming bedtime routine. Read him a story. Have him talk about his day ....fears, worries, and concerns if any. Ensure that he will be safe. If needed, keep on a small night light for him.
Try to limit watching any news show full of stories of death and dying or any other shows like that.
Additionally, you may ask your son's teacher if she/he has ever talked about kids being kidnapped and taked away and killed. Sometimes teachers may be teaching a lesson on stranger danger and bring those issues into the conversation without really realizing that they may do more harm that good.
The reason why your son feels so afraid after a vacation is because he is experiencing separation anxiety which is completely normal. He has been with the family for an extended break and must reacclimate himself to the routine of going to school. When you take him to school, drop him off and leave quickly. Hanging around, saying goodbye, and making sure that he will be alright will not make the situation any better.
Hopefully I have shed some light on the situation. If you need more help. please let me know.