He was changing. Naruto's personality had been in a downward spiral since his mission to the Land of Waves. It was subtle at first, but now Minato doesn't even recognize his son after seven years. Hell, he barely even sees his son. After he transferred to the ANBU, Naruto rarely ever came home, and even when he did, he was cold, distant, and angry. His mission reports had become increasingly concerning. Naruto's new apathetic attitude toward his team, friends, sensei, and even his father worried the Hokage more than anything.
An emotionless Shinobi was never a good thing, and it was even worse for a Jinchuuriki.
Itachi Uchiha was the only thing that seemed to help for a while, but it was false hope.
When Minato learns that Naruto was the sole perpetrator of an attempted fratricide incident during a crucial mission in Iwa, he is forced to confront the issue. But at this point, Minato is certain that his son might be too far gone to be saved.