I suppose you're referring to the following exchange (from here).
悟 ：えっ？お・・・お母さんって言ったでしょ？(I said "Mom.")
In Japanese, a large majority of kids call their mother either okaasan or mama, depending simply on the custom of each family (and switch very rarely). A kid ( <12 years old, say) saying kaasan is relatively rare, that is why it sounded weird to the mother.
So Satoru apparently switched from okaasan to kaasan at some point when he grew older. He first used kaasan due to his 'current' (i.e., as a grown-up) habit, but it sounds unusual to the mother, then he 'corrected' it to okaasan, which should have been the way he called his mother.
If I remember correctly, Conan (or Shinichi) the detective called his mother kaasan, but he is 15 or 16 years old. As people grow older, use of kaasan becomes more common, while it is by no means strange that a 50-year-old man calls to his mother by okaasan.