How do I stop paying child support for my 21 year old son?

Your Circuit Court website should have a form available for download called an "Affidavit of Emancipation". If the custodial parent (the recipient of child support) executes and files the form, it speaks for itself and will terminate child support. If the non-custodial parent (the payor of child support) files the form, the other parent may agree, or may file an objection calling for a hearing. The law in Missouri is clear, however, that child support stops at age 21 (unless the child has special needs), so there should be no dispute. If you are still paying (through income withholding, for instance), then you may want to hire an attorney to address the issues of overpayment and reimbursement.