The couple, who tied the knot in a private ceremony in Hawaii in 2010, are said to be excited after learning the unborn baby's sex, reported Aceshowbiz.
Fox and Green found out earlier this month that they would welcome another son to the family, revealed a friend of the couple.
"They're both really excited," the source added.
The "Transformers: Revenge of the Fallen" actress got pregnant with her second child when her son Noah was just 10 months old.
Earlier this month, Green said that he did everything Fox asked to make her happy.
The "Anger Management" actor, who also has a son from his previous relationship with actress Vanessa Marcil, confessed that he did not know how to juggle a job with young children.
"We honestly have no idea and we discuss it daily and think, 'How are we going to do this?' I think we'll see when it happens. People do it all the time.