He signed neither the Declaration of Independence or the US Constitution. It is very likely he never lived within the authority of the United States of America.
So why the title? He, and his young step-son, selected the site and founded what was to become one of the larger cities in the American Midwest. St. Louis on the western bank of the Mississippi River.
Trade. Business. Fortune. These were the forces which drove a lot of exploration in North America. So traveling the great river north to establish a fur trading post would have been a natural thing.
They chose the site for the trading post, village, city well. Moving south from the marshy, low lying land of the confluence of the Missouri and Mississippi Rivers, they established it on a bluff, with access to the best transportation system available without the hazard of frequent floods.