Grant announced the news on Twitter Saturday, reports contactmusic.com.
"In answer to some journalists, am thrilled my daughter now has a brother. Adore them both to an uncool degree. They have a fab mum," Grant posted.
"And to be crystal clear. I am the Daddy."
The 52-year-old and Hong first became parents in 2011.
Details of the newborn's name and date of birth have not been revealed.