The Tempest Lesson Plan

In Act I, scene ii, line 439, Ferdinand mentions a son of Antonio's who was aboard the vessel that crashed. Why have we not seen or heard anything about Antonio's son since?

In Act I, we hear about this son of Antonio's. In Act II, we see Antonio and his party but there is no sign of Antonio's son, nor is there any mention of him. (This isn't just in Act II, we never hear any mention of Antonio's son anywhere in the play.)

This is even odder when one considers that King Alonso (who is a member of Antonio's party) spends most of the play mourning the loss of his son, Ferdinand, who is separated from the group and presumed dead. Yet, Antonio never mentions his own son, who...