Welcome!
It's worth noting that, even if the FW subtest is a low estimate, for whatever reason, the remainder of the FRI cluster suggests that there is a genuine personal weakness in fluid reasoning (although possibly not quite as severe as FW would make it appear)--which does have some direct predictive connections with math achievement. So the available data, even if possibly not of the quality one would optimally like, is consistent at both the cognitive ability and academic achievement levels. And to your concern about directions on FW: the directions are quite clear in explaining how to solve these puzzles using the completed balance. If the examiner used standardized administration procedures, it is unlikely that your additional training on the task was qualitatively different from that provided during the formal test, especially since your DC's original explanation of the solution process is accurate.
If you wish to complete other FR subtests, there are two remaining on the WISC-V: Picture Concepts and Arithmetic. The former is a visual classification task, somewhat analogous to Similarities, but with images instead of words. The latter is a series of increasingly lengthy and challenging word problems that are presented and completed orally (no paper and pencil), and likely would not generate high scores, given his personal weakness in auditory working memory (Digit Span).
And to offer some perspective: none of his achievement scores are low. They are either perfectly age-appropriate, or above average. Please do not be misled by the grade equivalents, which are not truly anchored to grade levels. Any standard score 90 or above is at least age-appropriate. His strength area in reading comprehension is quite as expected, given his comparable strengths in verbal cognition.
Continuing, his cognitive profile is not inconsistent with being mildly gifted in verbal reasoning and in visual spatial thinking, but with some personal weaknesses in abstract-fluid reasoning--a profile which often means it may take him longer to adapt to new concepts and situations, and to shift strategies when the initial attempt is unsuccessful, but that hands-on exploring and experimenting helps him to problem-solve, especially once he sees how something connects to his experience and relevant applications.
