In the pic, Hirsch's son looks into the camera with big blue eyes that resemble the actor's.
The Into the Wild star and a former fling welcomed baby Valor in November 2013. Hirsh's relationship with the baby's mother is still unclear.
The 29-year-old actor told Us Weekly in December that fatherhood is "such a special experience that is really hard to put into words."
"He's just adorable," the actor said of Valor. "He's like an eternal campfire, you can stare at him forever. He's just beautiful."