What do I do if my ex found out that he is not the biological father of my6 year old son so he recently stopped payments?

This would depend on several factors. First of all is your ex paying voluntaruily or by court order from family court or divorce court. If the obligation is a court order, then he has the obligation to go to court to seek to cancel it by filing a modification petition putting paternity in question. If the ex is paying voluntarily and without court order, then you will have to file for child support and he then can raise the issue of paternity not being established. Another issue is whether the ex is equitably estopped from seeking to vacte paternity, This means that even though he may not be the biological father of the child, that he has acted as the child's father all the child's life and the child will be greatly traumatized by the truth as well as the lack of support. In this case he may still have to pay. Another factor with this issue has to do with your conduct as to when and how did he find out and whether you knew and kept it from him as well as your reasons why.  If you have a court order, you need to enforce the court order and if not, you will have to take him to court to make him pay. Either way, the issue of paternity will have to be addressed at some point.