Nope. According to Colorado statutes, you are correct unless it states otherwise in your order. He needs to file in court to lower the amount, and he should not be able to collect anything from you because he neglected to do so.
http://www.michie.com/colorado/lpext.dll?f=templates&fn=main-h.htm&cp=
(13) Emancipation. (a) For child support orders entered on or after July 1, 1997,
unless a court finds that a child is otherwise emancipated, emancipation occurs and child support terminates without either party filing a motion when the last or only child attains nineteen years of age unless one or more of the following conditions exist:
(I) The parties agree otherwise in a written stipulation after July 1, 1997;
(II) If the child is mentally or physically disabled, the court or the delegate child support enforcement unit may order child support, including payments for medical expenses or insurance or both, to continue beyond the age of nineteen;
(III) If the child is still in high school or an equivalent program, support continues until the end of the month following graduation. A child who ceases to attend high school prior to graduation and later reenrolls is entitled to support upon reenrollment and until the end of the month following graduation, but not beyond age twenty-one.
(IV) If the child marries, the child shall be considered emancipated as of the date of the marriage. If the marriage is annulled, dissolved, or declared invalid, child support may be reinstated.
(V) If the child enters into active military duty, the child shall be considered emancipated.
(b) Nothing in paragraph (a) of this subsection (13) or subsection (15) of this section shall preclude the parties from agreeing in a written stipulation or agreement on or after July 1, 1997, to continue child support beyond the age of nineteen or to provide for postsecondary education expenses for a child and to set forth the details of the payment of the expenses. If the stipulation or agreement is approved by the court and made part of a decree of dissolution of marriage or legal separation, the terms of the agreement shall be enforced as provided in section 14-10-112.