What does it mean when you hear your son say mom?

Assuming the question is about a dream and that the dreamer is the mom...

.... the underlying meaning reflects the dreamer's concern for the son's well being, and her feeling responsible to promote that well being in any way she can. A more specific interpretation requires further information, such as whether the son is still a child living at home. If one awakens immediately after hearing the child call, the natural response would be to get up and make certain the child is well and sleeping peacefully. Not only could the dream represent intuition of a problem, but the child might have actually called for his mother.

... Alternatively, if the son is living on his own, the dream could illustrate the mom's natural concern and anxiety about his living on his own (or being deployed in the military, or the dangers of living on campus, etc.) Although any actual problem is unlikely, the dream is a convenient and appropriate excuse to call the boy and express one's love.