Another way of thinking about this is to remember that IQs, though we tend to think of them in absolute terms, are only approximations of the true ability of an individual, affected by many small moment-to-moment, day-to-day factors that we collect under the descriptor of standard error. This is why they are most appropriately reported in ranges (formally called confidence intervals). Typically, index-level scores (FSIQ, GAI, VCI, VSI, FRI, etc.) are reported with 95% confidence intervals, which, for the WISC-V GAI, spans about 9 or 10 points (similar for the other WISC-IV and -V index scores). This is why a 10 point drop in FSIQ is not a huge deal, especially when moving from one edition of a test to the next. The 20 point drop in GAI is a bit bigger than might be expected based purely on standard error, but not completely unreasonable taken in combination with the factors I listed previously.
Secondly, it is worth considering the arbitrary nature of the definition of PG. Falling slightly below an artificially-determined cutoff doesn't mean this intellect is qualitatively different from one measured at two points higher. (And again, remember that the confidence interval may very well stretch to above the line, anyway.) If we choose to classify intelligence, which appears to occur on a continuum, we have to draw lines somewhere, not necessarily for intrinsic reasons.
Finally, psychometric reasons for score variations aside, why does it matter if he is MG, HG, EG, or PG? Will it make a difference in placing him appropriately, more so than his demonstrated academic needs (based on performance, intellectual satisfaction/challenge, balanced development)? Would you pull him out of his current classes if he were successful and happy in them, just because his number had fallen a little? Conversely, would you push him to be stay in classes where he was clearly floundering and stressed, just because his number said he "should" be able to perform at that level?
At the most simplistic level, does Davidson kick kids out if their scores fall? I doubt it.
