A man is paying child support for three kids but one is not his. Can he stop paying for that one?

He can if he goes to court, asks for a DNA test and then the court orders a modification or the child support order.

In Nevada once you are judicially determined to be the father, he would have to file a motion with the court to try and undo his support obligation to that child. If he marries and has step children, he is not responsible for those children and will not be a "break" in child support. He might get a $50 to $75 break should he go on to have other biological children.

In SC, a parent that is paying child support can claim a credit for other children living in their home only if the other children are natural or adopted children living in their home. A parent cannot claim step-children, unless there is a court order establishing a legal responsibility for that parent to do so. As far as the one child that is not his, He would have to file an action for modification of support due to a substantial change in circumstances and then prove to the Court that the one child is not his which would likely be considered the substantial change of circumstances needed to modify the child support. I do hereby clearly advise against any reliance on this information as advice or the application of it to a specific situation without a more thorough consultation with counsel.

He is not responsible to support your 4 kids. The father of those kids and you have the responsibility to support those kids. As for the child he is supporting but not the bio dad of, he may have been the only father that child has know and therefore he will continue to help support that child.

The short answer is No and No. Before child support was awarded, you were asked if the children were yours. If you had said that one was not yours, the court would have ordered a paternity test. If you stand "in loco parentis", in the place of the parent, then there is usually no way to retreat. You knew that you had three children to support before you agreed to support the other four. The four children should be receiving support from their father(s). Do you really want to be legally obligated to take care of seven children, none of whom are yours?

If he is adjudicated the father of all three, then he pays for all three unless he can convince the court the adjudication was by fraud and that it wouldn't be harmful to the child to have him not be dad (though the % of income is 25% for two kids and 29% for three so it isn't a huge reduction). No, he has no legal obligation to support his new wife's kids and it will not lower his support.