If it's a Missouri child support order, it will continue to be a Missouri case. Enforcement will be handed off to whatever state he is living in, but they will enforce the Missouri order, and will not have the power to modify it.
As for finding him, contact a private investigator. Check around on the internet. I know it sounds extreme, but attorneys use them all the time, and rarely pay more than $200. They search electronic databases and data mining sources. They don't have to fly to Texas to find him themselves.
This advice is based upon limited and hypothetical circumstances. For an answer that is specific to your situation, please consult an attorney. The answering of this question does not create an attorney/client relationship, and the poster should seek additional information from qualified legal counsel. Many attorneys, like ours, offer no-cost consultations.