My thoughts are that the WPPSI may have been insultingly easy for him and he just would not engage, and the tester may have been unable to tell that is what was going on. I watched my DD do the WPPSI at 4yrs9mths and could tell she was doing well but was also really puzzled as I kept waiting for the interesting or hard bits... She was pretty disengaged and did much better on the SB5 administered by a tester that gets gifted kids - because she was happy to participate.