How is his learning/acquisition of new information on the spot? A big part of Coding is learning what symbol is associated with what number in the key. In other words if one has to refer constantly to the key, he would be too slow which would get him a low score. In order to get a good score on Coding one should not refer to the key but should have learned the associations so that the key becomes unnecessary.
By this token, someone who has a low Coding score might have difficulty on memory such as the California Verbal Learning Test or some other memory tests: (Weschsler Memory scale, etc.) It's worth checking his memory.
