From NetFlix:

In this film based on a David Gerrold novel, John Cusack stars as a
recently widowed science-fiction writer who adopts a 6-year-old boy
to quell his loneliness. The catch? The kid ( Bobby Coleman) claims
to be from Mars. At first, the new dad doesn’t pay much attention to
the boy’s story, but when an odd series of events occurs, he begins
to believe his son may be telling the truth in this film co-starring
Amanda Peet and Joan Cusack.

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This is NOT science fiction. My all-time favorite movie about a child is “Dear Frankie”. If you have not yet seen “Dear Frankie”, I urge you to do so. However, “The Martian Child” is probably the most gentle movie about a child that I have ever seen. John Cusack plays a widower who does the most wonderful job trying to bring the boy into a normal interaction with the rest of the world. It is sort of a model about how to raise a sensitive and unusual child.