You will need to look at the child support order to see what it says about the arrears, but there are a number of remedies under the law to collect those arrears. You would need to have an attorney review the order and explore with you about different possibilities, such as reducing it to a judgment and executing it against any property that he has. You many need to work within the court file that already exists.
Because there are a number of possibilities, you need to look at whether any of the options available to you will result in you receiving anything for your effort. A private attorney may be able to work with the child support agency to help collect arrears. You will want to consider whether pursuing the arrears will actually help you collect anything so that you aren't spending money you will never get back.
Finally, you will need to consider the statute of limitations. With the amount that is owed, it would seem that these arrears may go back a long time.
The information you obtain from this post is not, nor is it intended to be, legal advice. No attorney can give legal advice without a full consultation wherein the attorney is able to get as many facts as possible. Any information contained in this post is for informational purposes only. Please seek the advice of a competent attorney in your area to get legal advice specific to your facts.