Answer:
reticular formation
Step-by-step explanation:
You tell him that his son's brain is still maturing and has not fully developed, and that different areas of the brain such as the prefrontal cortex, reticular formation, hippocampus, and the corpus callosum develop at different times/stages of the childs growth.
The child should be able to sit through the entire story time once the reticular formation develops more fully. The reticular formation functions as a filter for inattention to repetitive meaningless stimuli, and once this portion of the brain is more developed in the child, he will be able to filter what is meaningful from what is meaningless and hence will be willing to sit more longer for the stories.