G.B.
Geez, this happens on TV all the time so it must be true...lol. But really, he is not legally these childrens custodial parent. Even if he is on the birth certificate. Marriage makes that a legally binding issue. Have an attorney draw up legal papers, have them signed, and then filed. I am a grandparent raising grandchildren and laws are changing to favor these type of issues.
For example (not a real instance, my daughter is alive), if I had a daughter that passed away who had children and the father was a good upstanding guy but not married to her I would have the legal right to file for guardianship and then file to adopt the children because the mother is dead. The father would have to file an injunction to stop the proceedings and it could last for years with me having the kids and him fighting for them.
It is better to do it legally.