Check Colorado's Rules of Civil Procedure 4(g):(g) Other Service. Except as otherwise provided by law, service by mail or publication shall be allowed only in actions affecting specific property or status or other proceedings in rem. When service is by publication, the complaint need not be published with the summons. The party desiring service of process by mail or publication under this section (g) shall file a motion verified by the oath of such party or of someone in the party's behalf for an order of service by mail or publication. It shall state the facts authorizing such service, and shall show the efforts, if any, that have been made to obtain personal service and shall give the address, or last known address, of each person to be served or shall state that the address and last known address are unknown. The court, if satisfied that due diligence has been used to obtain personal service or that efforts to obtain the same would have been to no avail, shall:
(1) Order the party to send by registered or certified mail a copy of the process addressed to such person at such address, requesting a return receipt signed by the addressee only. Such service shall be complete on the date of the filing of proof thereof, together with such return receipt attached thereto signed by such addressee, or
(2) Order publication of the process in a newspaper published in the county in which the action is pending. Such publication shall be made once each week for five successive weeks. Within fifteen days after the order the party shall mail a copy of the process to each person whose address or last known address has been stated in the motion and file proof thereof. Service shall be complete on the day of the last publication. If no newspaper is published in the county, the court shall designate one in some adjoining county.
You really need an attorney to assist with this. You have to provide an affidavit, showing you exhausted all means of locating him, and you must strictly follow procedures.