You can find online how to calculate the GAI for the WISC, but not for the WPPSI. I just calculated my dd's score using those tables, and the results I find online are one to 4 points higher than what she has (we have that calculation done using 2 different subtests for one of the indices, so I can play with both).
My guess from your ds' scores and extrapolating from my dd's is that he'd have something in the low 140s, and percentiles 99.6 to 99.8%.
Your tester should be able to calcuate that for your ds.
