I would be having some discussions about what it is that's happening when he's in bed that makes him scared. (For eg, our DD was scared and it eventually turned out that her nightlight cast moving shadows because of the colour-changing bit rotating). Instead of just saying monsters aren't real, explain about stories and how people have invented monsters to make their stories more interesting. Also, explain about dreams. It's hard to tell a kid that monsters aren't real because if they are in your dream they sure seem pretty real. So a talk about dreams and how it seems real but it's just a kind of story that goes away when you wake up. Talk about the nice dreams he has. Talk about when he's going to bed, he can think about nice things that make him happy and that will help him to have nice dreams. Talk about how nice his bed is and his bedroom and find out if there is something he could have that would help him feel happier. It might be leaving the door open a crack, or leaving a very-low wattage lamp going, or listening to a story or some happy music while he falls asleep.