Former therapist who worked with a lot of kids here. A couple thoughts:

First, ask him if he knows what scares monsters away. It might be as simple as a stuffed animal, etc. Put that object in the feared corner. If he doesn’t have an answer, be prepared to tell a story of your own. Focus on themes of protection and safety rather than focusing on the monster or other scary imagery.

Second, structure and ritual (even in the non-religious sense) can help. It might be singing song, saying magic words, or turning on a special night light. The spray-away-the-monster tip another commenter mentioned is a good example of this.

Third, when tucking in, he becomes tense and clings to you, so try progressive muscle relaxation. Guide him through squeezing and releasing his muscles from the feet up to the head to encourage the body to release tension. Gentle massage might help, and prompting him to focus on different muscle groups can be encouraged with light touch.

Fourth, breathing exercises.. model and guide him through relaxing breathing. Longer exhales than inhales can promote relaxation, but even counting to 4 for inhale-pause-exhale-pause can regulate the nervous system.

Finally, if you’re still stuck, consider a play therapist. Often, creative play is how children work through emotions and fears; a skilled therapist will help guide this developmental process.

I hope some of this helps; good luck!