Are separation papers legal in nys for non married couple with twin babies

Unless the couple was married or had a registered domestic partnership, they need a child support/custody and visitation agreement, not a separation agreement. Although such a written agreement doesn't necessarily need to be done in court, if your son is concerned his ex won't follow it unless it is a court order signed by a judge, he should start a custody/visitation proceeding and he'll ultimately get a court ordered visitation schedule. The same goes for child support, but if he's the non-custodial parent he'll be paying the mother, so if she wants a court order she should file the support  petition.  Child support payments are set by law and calculated based on salary and the number of children. He's should minimally check with a lawyer to make sure he's paying the correct amount.