Alma clearly hears the Spirit telling him that he must reprimand Corianton, in the strongest possible terms. Like most loving fathers, Alma would rather praise his sons, as he has with Helaman and Shiblon.
The Lord’s commandment is generic, not specific. Even though Alma has only had one son who has led people away through his bad examples, any or all of his sons might do so. The command applies to all, but Corianton is the reason the command is necessary at this time.