Garner, 39, is currently expecting their third child and it's claimed a little boy will be joining their older two daughters - Violet, five, and Seraphina, two.
Affleck is said to be excited about having a 'little sports buddy' in the family.
A friend said to Life & Style: 'He can't wait to take his son to baseball games and play catch with him.'
However Garner is said to be a little more nervous about welcoming a baby boy.
A source said: 'She feels like she won't know what to do. She's close to her two sisters, and her instincts were spot-on with the girls. But she never had a brother and doesn't know the drill.'