I’m not sure what Congress can legally do, pass a law saying that Texas cannot execute this man? Again, the US Supreme Court has already ruled on this matter and told President Bush that the Federal government cannot interfere with how Texas enforces its own laws.
If Congress enacted a statute which implemented the treaty, then it would supersede the Texas courts. That is what the SCOTUS found in its judgment on this case. The fact there currently is no such statute is why Texas did not have to adhere to the World Court ruling.