Great to hear that you have taken him to a vet. Maybe you can take a straight line measure of him.

Another hint is the color of his plastron. If it has any black, then it's a mix, or a Gulf Coast Box Turtle.

The tan phase of the Gulf Coast can have all tan plastron. What sometimes gives them away is the size of the turtle.

Your definitely looks like a three toed though.

It is probably just an inter grade.