We even gave him a tape player loaded with soothing music, but he still came downstairs and told us that monsters were reaching up from under the bed. We talk to him calmly about his fears but that doesn't help. The Potter books teach children that some people are good, some are evil, some are stupid and bigoted and not to be endured. Most books for older children don't upset the younger ones, because they can't follow the plots, but J.K. Rowling writes so well and so clearly that her adventures may upset sensitive children. If a child is super-sensitive, however, she should probably be a little older, particularly to read the later books, since each new title is a little darker than the last.
Every chapter is a cliffhanger but the books produce no monsters.
Monsters hate soap. If they still reach out for your son, grab a broom, chase them out of his room and out of the house, fussing like a harridan all the while. Your son will laugh but he will also be amazed at your audacity. Anything that makes your child feel safe will be good for him, but have him sleep in his own bed, instead of the couch, because children need to know that they can put themselves to sleep. That actually makes a child feel more secure than having a parent nearby.