Unless you know for sure that he was in an orphanage, a child could be adopted without being in an orphanage. You could start by contacting the state of Missouri that should have every record or every adoption in the state. They should know what county the adoption was finalized in and should direct you of what to do from there. Every state has different adoption laws and some change depending on what year born, etc. records as far back as 1910 might be open. I also know that Mo has an adoption registry.