Many states provide for automatic termination of child support, but if support is wage deducted, some employers still require a termination order. Go to court on a motion with sufficient amount of lead time to secure an order of termination which is effective on the date your son becomes emancipated.
The order obligating you to pay child support should include a provision stating when that obligation terminates. In most orders, that is when the child turns 18, unless he or she is still enrolled in high school - in which case the child support obligation continues until the child is finished with high school. Assuming that language, or something like it, is in your order, your child support obligation should terminate automatically.