I read the subject line and thought "I bet he's four". This is an absolutely classic age for developing fears.
I don't have any quick answers, but it is something that many children go through around this age (my DS isn't having exactly the same fears as yours, but he is having frequent bad dreams, generally about DH or me leaving him forever and going to live somewhere else, that leave him very distressed) and it will pass in time.
yes, same here too - have to keep a low light on while he drifts off and a light on during the night in case he wakes up. We're constantly having to reassure ds that nothing will come in through the windows and there's nothing scary about his room or the house and it's our home etc., etc but he's still worried if it's dark or if there are shadows so leaving a light on is an easy enough remedy.
My little girl is almost 2 and has suddenly got scared of the dark too. My theory is that it started when the clocks went back recently and it was dark walking home from nursery, and crucially, when she went to bed. A Hallowe'en outing to see fireworks didn't help at all, so now she cries at the dark night and needs to be reassured there are no more fireworks. Poor pet. So I've put in a little plug-in light to give a bit of a glow.
I hope it passes, but to be honest I hate the pitch dark too, and need some kind of light when I'm in bed, even through the crack of the door, or from a street light. I'm 38.
DSD started crying in the night frightened of monsters and the dark from the age of 4. We asked DSD where the monsters were and what dark places she was frigtened of. She showed us in her wardrobes, under her bed and so on. We bought DSD some fairy dust (confetti type stuff you can buy from Tesco) and some fairy lights. We arranged the fairy lights on her headboard. We then went round her room. Firstly sprinkling fairy dust to shoo the monsters away in all the dark scary places and then leaving them a little present to show them we were really their friends. DSD was then allowed to sleep with her fairy lights on low. She was happy to try to be friends with the monsters but reassured that the fairy lights and fairy dust were also there to protect her just in case. Job done, not had any problems since!
DD 2years 3months also has a light on low all night, guarded by a veritable army of teddies! No probs so far with her, but I can see us investing in more fairy lights and fairy dust as she gets older.