In Missouri it is very difficult to have your parental rights terminated unless there is just cause or someone is stepping in to take the role of that parent. If you signed paperwork giving your ex wife physical and legal custody that does not mean you have had your rights terminated. It sounds like you have agree that she will have sole legal and physical custody. You still have rights to your son, including visitation.
No, it does not mean that you signed your rights away as dad. By giving her sole legal custody, you've basically given her the sole authority to make major decisions regarding your son (i.e. major medical decisions, religious training, counseling). Sole physical custody means that she will be designated as residential parent for education and mailing purposes, and it simply means that she will have the vast majority of the physical custody of the child. As Molly states, it does not mean that you cannot have visitation with your son and it certainly does not mean that you are terminating your rights as the child's father.
This answer is for informational and educational use only. This answer does not create attorney-client relationship. For more details, I recommend a private consultation with a family law attorney. Stange Law Firm can offer you a free one hour consultation if you call (314) 963-4700.