We simply kept reassuring that there was nothing to be afraid of in the dark, but put a small night light in his room, DD has to have a light on in the hallway when she goes to sleep and we turn it off once she's asleep. DS also has a Groclock that reflects some light. I never liked sleeping in the dark until I was 13 so have no problems putting a small light on that reassures them.

ETA: Maybe a Glow worm or a light that he chooses? Ikea have some cute ones and it could become a novelty factor that he has to stay in his room by himself to get the special light on??