Fears are very common in children between the ages of three and eight years of age. Often, they are triggered after hearing a scary story or seeing a movie, as in the case of your son. Children this age have a growing awareness of death and their own vulnerability. They feel small and powerless in a big world that is sometimes confusing and frightening. Furthermore, many children have a vivid imagination, which further contributes to their fears. Monsters become a tangible symbol for everything that is confusing and scary and that might harm children.
Fears are aggravated by anything that poses a potential threat to children's sense of security, such as moving to a new home, starting a new school, parental stress, disputes, divorce, any kind of abuse, illness or death in the family, a mother's pregnancy, or the birth of a sibling. So if there is any unusual stress in your son's life, that stress would help to explain his fears.
I personally feel that movies should be screened very carefully for young children, because there are often scenes of separation, loss, danger, or violence, with frightening creatures. Most movies marketed to children are not appropriate for children under the age of seven or eight years. But please don't blame yourself for letting your son watch ET. You had no way of knowing how it would affect him.
The fact that your son is afraid of monsters does not mean that he will grow up with phobias or anxiety disorders. Most children eventually outgrow these early fears. Fears such as these indicate that your child is sensitive and has a vivid imagination. These are very desirable qualities that are the basis for compassion and creativity. Keep in mind, also, that as your son grows older, you will want him to have a healthy sense of his own vulnerability. Some teenagers appear to be fearless, and they engage in risky behaviors. Perhaps they were told too often as children that there was nothing to be afraid of.
Most children feel less fearful when a parent is nearby. They have a legitimate need to be physically close to somebody who will protect them against all imagined dangers. Forcing children to stay alone in their room when they are terrified does not help, and sometimes only aggravates the situation.