Here is what is important...is your son an American citizen? If he is , he does not need a passport to travel to Mexico by LAND, but he does if he will be traveling by air. American children under 16 may travel to Mexico or Canada by LAND only with just their birth certificate. If they are going to travel by Air , they must have a passport. As a U.S. permanent resident YOU just need your green card...and if the baby is also a green card holder, he would need just his green card. See site below and read section under "U.S. Lawful Permanent Residents."