I am not aware of any way to eliminate your arrears, besides your ex-spouse formally waiving them. It should be noted that if the arrears are reimbursement for State benefits to your son, you may be able to negotiate a settlement with the State on these amounts.
My recommendation would be that you obtain a free consultation from a family law attorney to review your case before taking action.
Judd S. Nemiro
Law Offices of Judd S. Nemiro, PLLC
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You won't be able to eliminate your arrears payment, unless Mother agrees to waive that - which sounds unlikely. However, you can file a Petition to Modify Legal Decision-Making (formerly custody), Parenting Time and Child Support with the Court to address the change in circumstances. I'm not sure from your message, but you may have already done that. If so, I'm unsure why the case is being dismissed. You can file a Motion with the Court to request that they extend the dismissal date or that they set the matter for a Trial regarding your Petition to Modify (assuming you've filed one).