"brother was married and had three kids...he was hit with $500/mo child support payment"
$500/month for three children is very low.
"It has been four years and his exwife hasnt made any mention of the child support payments"
What HAS your brother been paying in support? If he's not been paying anything at all, then it could be quite possible that he could face serious legal sanctions, including jail time, (not to mention that those amounts keep accruing, so the debt is continuing to grow, interest accrue, etc.).