My fiance has a child that was just born, She wants him to be a part of the babies life and he wants to sign his rights over because he says finacially he cant do it. We have two children already. I do want him to be a part of the babies life and be there for her. If he decides to sign his rights away can the mother fight it and make it where he cant do it?

Its not that simple. Your husband can not just sign away his child. In fact unless the Mom is married and her current husband wishes to adopt child its impossible. Your husband does not have to have a relatinship with child (if he chooses) but he cannot avoid his obligations

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Your best advice legal advice does not come from internet message boards, Police, friends, family, your neighbors etc.It comes from an Attorney. Advice given here should not replace legal advice from a qualified Attorney.
Non legal parenting and relationship issues http://www.parentnook.com/forum/

I agree with underdog. Have you browsed through the information in LawInfo's Free Legal Resource Center to learn more about your issue yet? See: http://www.lawinfo.com/consumer.html and http://resources.lawinfo.com/en/index.html. You can certainly try to speak to a lawyer to determine what legal options may be available. In the meantime, you may be able to learn more on your own. Search the "Free Legal Resources" tab, or browse the Consumer Resources. Good luck.