Divorce and Child Support

My fiance was divorced from his ex wife over 7 months ago. In the divorce papers, they agreed to child support and custody/visition. My question is, since child support is in the divorce papers is this the same as child support order? They never went to a judge to get a child support order, it just had it in the papers. Here is the situation: the ex-wife is a total money hungry women. My fiance pays $600 a month on 3 kids, but one of the kids isn't his biological son. For one, at the time of the divorce he could afford $600 a month b/c he had no bills and lived w/ a friend. We now live together and share bills. He is having a hard time paying child support especially on another kid that isn't his. Is it possilbe to get a child support order (not like the one in divorce papers) that can lower child support? Also,  the one son isn't his but at the time the divorce agreement was made, he said he would pay on him. He was told by her that she would find the boys father and once she did he didn't have to pay anymore on that boy, well, now she refuses and thinks he should continue to pay. Is there anyway he can go back and change it to pay only on his 2 biological sons even if in the divorce papers has that he would pay (he thought he could trust her about finding the boys father so they didn't put a clause in the divorce papers about stop paying after he was found). It just seems that if he isn't his biological son, he wouldn't be held to pay child support. He was only w/ her for 3 years (the boy was 2 years old when they got together and 5 when divorced). Please help! This lady is a total waste. The children aren't clothed well (holes in clothing/dirty), she has told us she needs the money for gas, cigrettes, etc (things that are for her, not the children), she was also getting food stamps (so the money he gives her is going in her pocket instead of grocries) and the kids aren't bathing. She is a poor excuse for a mother and harasses my fiance daily about child support even when its not due. Please, any info would be gratifully appreciated!! We are taking her to court as soon as we can b/c this has gone on too long and it's only been 7 months since the agreement, it will continue to get worse.

OK, through information and also people I have talked to, to get their sides of the story, child support can be changed, especially if there is change to ones life, bills, job, etc....but it is going probably going to take going back to court, because she is going to want to fight it, remember the first consultation with a lawyer is free, so going in to set up what you should do, and see what the lawyer says, does not cost, it cost to carry through with it all.  Also I do have a question that is important, actually 2, is he on the birth certificate at all?  Did he adopt the child by signing any papers?  Those are important, because if he did, he is more then likely going to have to pay for the child, if not may be able to get support dropped a lot lower.  Who I feel sorry for in this situation is the children, it is sad to have to see children grow up that way, and my heart goes out.  ALSO, have either of you thought of showing she is unfit mother and getting full custody?  There are just several things to be answered here, but yes child support amounts can be changed, but he will have to have proof of everything that has changed in his life financially.  I will also get a few more opinions for you and look through my information.  Also I may be stepped away for a couple days, from a computer, so do not worry, if you do not hear right back from me.