If all of the Subtests of the WJIV that were administered are already listed, then you don't have enough subtests for the Gf-Gc. The GIA is derived from one Subtest from each of the core cluster areas, while the Gf-Gc requires two from each of the Comprehension-Knowledge and Fluid Reasoning clusters, which are currently represented by Oral Vocabulary and Number Series, respectively.
The BIA (Brief Intellectual Ability) has only the first three subtests (verbal knowledge, fluid reasoning, and verbal working memory tasks), without the speed and long-term retrieval tasks that you've observed are among his lower scores. That's probably the closest you can get using existing data. The GIA is generally considered a more robust measure, but comparing the BIA and GIA could be an exercise with some utility, at least for you and your child. I'd be hesitant to call it a strong argument with the school, though, since it's essentially a cognitive screener.
The Gf-Gc would need two more subtests.
