Jennifer's "monster spray" idea (we called it "dream spray") worked great when my daughter was younger.
One other thought: Could there be something in his room that to you or me would look fine but to him, in the dark, seems scary? Objects that are familiar in daytime can cast strange shadows, etc. at night. My daughter recently got scared of a cat poster that had been on her door for a while -- but the cat's glittery eyes, catching light from the hallway if the door was open at just the right angle, looked scary to her. Another time, a sweater thrown over the back of a chair seemed OK by day but at night was scary for her. Easy to fix! You might have to work with him on whether an open closet door, or things throwing shadows from his night-light, etc., might be a problem.
Please be careful not to belittle his fears or laugh them off -- it's very real to him. This is a pretty typical age for nighttime fears and loneliness, I think, and though he does have to understand that bedtime means bedtime, it's also not wrecking his routine to come to him if he's really distressed. It has helped my daughter, who's 7, for me to go into her room and look at whatever she says is scary and agree with her that "Wow, sure, that does look weird, and I'll fix it right now." Good luck.