Then there's either some underlying physical issue there or a curious mental block. It's not every day that horses walk backwards in the pasture, but I'd guess physical. Just curious, but how do you get him out of a trailer?
If it is something physical blocking him from being able to engage those muscles then it's unlikely he will be able to make much process in developing them. I'd get a vet out to make sure nothing is hurting, then teach him to back. And the other exercises too :)