All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If you marry that man and he has a son from another wife, then his son is a non-Mahram (marriageable man) to your daughter, and thus it is not permissible for them (i.e. his son and your daughter) to be in seclusion with each other or to mix together in a suspicious manner. Rather, they have to be considered strangers to each other.
Allaah Knows best.