Has anyone teased him about spiders, has he seen any movies out there on TV or PC, has he older brothers or family may have talked about them frightening him, it sounds as if something has happened.Have you asked him ,why and what triggered it, a lot of children and adults are afraid of Spiders it is a phobia, there are books written about it, google for information.Dont react to it just say there are no spiders there and let it go, if you make more of it he will remain afraid,
My son had a spider phobia at night too. We solved bedtime issues by spraying his room with bug spray (really air freshener) every night before bed. We kept him out of the room while spraying, so he would breathe in the fumes (hence going in with the two spray cans) and then he was none the wiser to the switch off. You could try that and see if it works for him. He also still uses a night light.