If you want to impute income you must show ability and opportunity. If you can show there are various not just one job that he qualifies to do then the court may impute the amount on him. He could always rebut the presumption by applying for those jobs and not getting hired.
This is just my opinion and not a comprehensive answer. You assume the risk because this answer may not apply to your situation depending on the facts.
First, you cant actually "waive" child support but you can agree to less than guideline if certain conditions are established. Further, if you agree to less than guideline you dont need to show a change of circumstances in order to modify support (go back to guideline) .
Second, you are describing "imputing" earnings or trying to prove the other parent can earn more than they are earning. Education, prior expereince, job openings and pay rates , can all be used to establish he could get a job and that the court should impute some earnings to him. You have to plead this and use documentation as described.
The answers on this discussion board are general in nature and NOT intended as legal advice. Responding to questions does not constitute an attorney-client relationship. Always see a lawyer about your individual situation.
I don’t believe your problem is so much proving up an earning capacity but rather collecting the money from him. He has apparently taken to the deadbeat life. He can certainly get a job, he can make $10 or $12 an hour at In and Out Burgers with benefits. If you go to Court and obtain an Order for support you have no wages to attach and no way to satisfy the Court Order. You have more of a practical problem rather than a legal problem.
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