First get a DNA check and make sure its your baby. Sorry to be so blunt.
All this stamp paper and promises are worthless from third parties. If wife is truly sorry, then ask her to sign on a stamp paper with her parents as witnesses along with some common friends as witnesses. The agreement must say that if she in any way whatsoever again, indulges in any kind of illicit conversations or behavior with male friends, she agrees to give her son to you for sole custody and relinquish all maintenance and other claims, including right to residence.
THIS YOU MUST GET as it is sign that she is truly sorry.