I am wondering if I need a prenuptial agreement if my fiance has past due child support. Can the state of WI take from my income?

No, they won't come after your income or your house if it stays in your name, but they may say that he has additional disposable income because of marrying you, making his support higher and limiting further the amount of money the two of you have.  A pre-nup would be a wise thing, especially since your fiance has already demonstrated that he's unwilling or unable to pay regular support for his child.