Not sure, if you're already using a nightlight, which is what I was going to suggest. Will share our tips anyway in case they might help...
Dd1 (3.3) is quite scared as well, but is ok with 2 nightlights in her room. We've just put up some glow in the dark stars, which she loves (ie is a positive thing about it being dark, she can see them).
We also don't shy away from reading books with monsters etc in (but not at bed time!), and talk about what to say to monsters that you're scared of (we tell her to shout boo, and scare them right back). There's a lovely book called 'emily brown and the thing', although it might be a bit old for your ds.
When she has a bad dream and is scared, I've tried sprinkling 'good dream' dust on her (all make believe, although some parents fill a squirty bottle with water and spray the room) - you could try a similar thing but to get rid of monsters or whatever.
You can also get nightlights that he can actually take to bed and cuddle all night.
Is there anything in his day to day life that's changed and could be upsetting him? Maybe the upheaval of Christmas?