Honestly, the question of his true ability is not one that can be answered based only on the information presented here. I would suspect that his quantitative reasoning skills are higher than the FRI currently suggests, based on his strong Ar score, but can't prove that using these data. ADHD could explain most of the lower scores, but it may also be that there are additional weaknesses in processing speed or fine-motor skills, which may feed frustration in school, especially at an output-oriented private school (which may contribute to the not-quite-diagnosis of ODD).
He cannot validly take the WISC again for two years, so that's not one of your options. Of comprehensive cognitives, the SBV is good, but showing its age a bit. You still have the DAS-II, WJIV COG, and KABC-II (also on the older side; it's undergoing a normative update). The only one with truly current norms is the WJIV COG, though I would take the DAS-II, also. Since the question is more his nonverbal/mathematical abilities, you could also try a nonverbal, such as the UNIT-2, WNV, or TONI-4, of which only the UNIT-2 is better than middle-aged.
Brief measures do have some caveats, but among them there's the WASI-2 (based on the WISC), RIAS-2/RIST-2, WRIT, and KBIT-2.
In any case, the decision-making process regarding schools also needs to include considerations related to his second exceptionality (ADHD), especially as there generally are escalating output and executive function demands in a a high-achievement placement. You may already have some information on how his existing school is likely to work with those aspects of his learning profile. The new school is, of course, an unknown quantity, but you can at least ask those kinds of questions. They may not live up to their claims, but if even their claims fall short of addressing his needs, then that will be telling.
