He explained: "Thomas has got no muscles, he's just sort flopping around. His skin's got this new baby sheen and he's just squishy and soft and squid-ey."
Jack - who is married to cellist Tanya Haden - also says his two-year-old son Samuel is jealous of the family's new arrival.
He said: "Samuel doesn't like seeing mommy feeding the new one - it makes him freak out a little bit."
The 'Nacho Libre' star revealed Samuel enjoys pretending to be a superhero.
He added to US TV talk show host Jay Leno: "He loves to wear his cape and run around the house and pretends he has super blowing power to make people go away.
Jack Black thinks his son looks like a baby squid.
