J.W.
Get a family lawyer and get into court RIGHT NOW. TODAY. Whether there is an existing court order or not--your son has lived in Missouri most of his life and Missouri has jurisdiction over him. But if you wait too long, Oklahoma will get jurisdiction. In most states, jurisdiction over the child is based upon the child's residnece for the previous 60 or 90 days.
That your ex did this while your son was enrolled, and without consulting with you about it, means he may also get the privilege of paying your attorney's fees. I can recommend a good family lawyer if you live in the St. Louis area.