I think the nightlight on a timer is a good idea, it does seem to be very common for a fear of the dark to kick off around this age, so I would expect it is real. As for how to deal with the monsters, well, a couple of ideas:
1) Tell him that he is the boss of his bedroom. No monsters can come in if he says so, and if they want to come in then he makes the rules. Then at bedtime you can make a big show of walking around the room 'bossing' all the monsters out - No monsters in this bedroom, you are not allowed, you're going to have to go out the front door and go away.
2) Put some lavendar oil in a water spray bottle and make 'Monster Spray' to spray around under the bed/around the curtains etc - it keeps monsters away.
3) In the daytime, encourage conversation about monsters and let him use his whole imagination as to how scary they could be - but also how funny they could be... so explore 'what if there were monsters in my bedroom'. Well, mummy and daddy would come and get them out! 'what if the monsters just wanted a cuddle?' 'what if the monsters were really scared of little boys?' 'what if the monsters gave you a secret chocolate cookie in the middle of the night?'