off hand.....i would think they might change the date bc more people wanted newborns. just a thought. i don't have any advice except to tell you i'm sorry.
i do have a friend whose birthdate is different than that on her birthcertificate....it is a long story about being born in a foreign country far from a hospital and by the time they arrived at the hospital it was a month later, and that is what her bc states. it always frustrated her bc she knows all of her docs (bc, driver's license, etc) are not right.
i don't think there is a way to rectify it, unless you can prove that it was wrong. i don't know if it is possible in guatemala, but i would try to see if i could get his original birth records if he was born in a hospital. i'd look into it....the fostermom oughta know when he came into her home, so i'd definitely investigate.