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No, a letter won't do it. To make him pay the amount he was ordered to pay and didn't you would have to file a Motion for Contempt and those are more than just filling out a form. You really need to hire a lawyer to help with that although the judge will almost certainly make him pay for your attorney's fees at the end of the case.
You can also consider contacting the Oregon Department of Justice, Child Support Division and asking for their help. Th einformation on how to contact them is at this page: http://www.doj.state.or.us/dcs/pages/index.aspx
They can file the Motion for Contempt for you and file to enforce the order. They also don't charge you a fee. However, they don't always take every case and they are usually a lot slower than hiring your own attorney. That slowness is offset by the fact that they don't charge though. Also, they can get access to his pay history, etc. so it's easier for them to show the judge that he was just refusing even though he was working.
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