There are not enough facts to answer this question. I do not know why he is filing the motion and whether it has any grounds. The way you fight his request is to answer by alleging there is no need for an emergency hearing and no reason to change the orders you just entered into. Tell the court this motion was filed only because he wants to reduce child support.
As the filing requirements for Ex Parte requests are different from county to county, I must assume that he has properly filed his Ex Parte request and has properly served you with the moving papers or at least has made reasonable attempts to serve you with the moving papers. Service issues aside, lets get to the meat of your question.
In terms of doing "what you can to fight him" it is necessary to define what exactly an Ex Parte Application actually is. An Ex Parte Application is a motion in which the moving party is asking for the court to order something that, if not ordered immediately, would allow the responding part to do irreparable harm. The risk of irreparable harm vitiates the request for the order and must be great enough to overcome the extreme prejudice that such an application has on the responding party. The prejudice I speak of here is the fact that the responding party in many cases gets no more than 24 hours to prepare for the hearing.
In light of this analysis, it is my opinion that attacking an Ex Parte Application is really three fold,
1. Attack Service: (Moving party made no attempt to serve me, service was too late, etc)
2. Attack the allegations of irreparable harm: (No irreparable harm exists to necessitate a hearing with only 24 hours notice to the responding party, should be a regularly noticed motion, etc)
3. Attack the merits of the request: (No grounds for the requested relief).
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