If you're trying to find his real parentage, then looking for WPA records might not help very much. I would think that he would just give the "official" information to the WPA, rather than the "real" info. (Would he even know the real info....?) Also, I doubt that he would need to provide his parentage....?
Maybe you should track down orphanages in SD first.
Also, have you tried to find a birth certificate for him? Where in SD would he have been born?