Question for those who saw the movie It's Complicated with Meryl Streep?

I just saw the movie and I remembered that Meryl and Alec were divorced for 10 years, and that he cheated on Meryl with his new wife.Her child is around 5 years old, or so, is he Alec's son?The movie made you think he was her child due to a prior affair.If he was around 12, I could understand.Also she wanted to have a baby with Alec and going to infertility clinic making me think that Pedro is not his son.Am I wrong, or is this an error in the movie?