There is no way that Alma had forgotten to teach his sons about the coming of the Messiah. It was Corianton who had forgotten, or perhaps not fully understood, what he had been taught. One of the most obvious signs of Nephite apostasy was the rejection of the doctrine that teaches the need for the coming Messiah. It appears that Corianton had either become convinced of that idea, or, at least, was on the path to believing that a Messiah was not needed. Therefore, Alma teaches of the coming Messiah.
Alma reminds Corianton that his ministry was to declare that very message to the people. That was the ministry, and perhaps the belief, that he had abandoned.