Liv, 37, who already has son Milo, 10, from a previous marriage, welcomed her baby boy with boyfriend David Gardner on February 11, reported Us magazine.
"When I got the call that Liv was going into labor, after crying like a baby myself, I immediately dropped everything and went to to the airport to get on the next available flight," Steven said in a statement from a hospital in New York City where Liv, 37, gave birth.
Steven also took to Twitter to express his happiness. "Love is love is love reflected another LivTyler moment (God Winked again)...So proud of my Alpha Pup," he added on Twitter of his eldest daughter.