Ah. The score you describe as FSIQ with only VCI and FRI was actually the GAI, which is calculated from the two VC, two FR, and block design subtests. That was a reasonable call to make, since his PSI (98) is well lower than VCI and FRI.
Also, the more manipulations he has to make in his head, the weaker his working memory scores become.
I wouldn't describe the change in his BD scores as a huge decrease. It dropped by three points when moving from the WISC-IV to the WISC-V, which isn't crazy, considering the phenomenon of norm obsolescence. And it's essentially the same on the WISC-V from three years ago as on present testing. Bonus points for speed matter more as you get older, which is probably sufficient to explain any changes that aren't just typical test variation.
