FWIW, your WISC-V nonverbal results appear to be relatively consistent with the OLSAT. The verbal portion of OLSAT is a very different test from the WISC VCI, as it consists partially of following spatial directions, and partly of quantitative reasoning tasks, orally-administered, which are more like the WISC-V VSI and FRI, respectively. IOW, there is very little on the OLSAT verbal that overlaps with the WISC-V VCI. OLSAT is loosely timed (some parts are rigidly so, others are examiner paced), so it might benefit someone with high average processing speed (which is good, BTW).
If alternate measures of intelligence are options for access purposes, you might find that the composite number is more reflective of his VSI and FRI if you ask for a Nonverbal Index to be calculated, which would include VSI, FRI, Picture Span from WMI, and Coding from PSI, and exclude the VCI, and the Digit Span subtest of WMI. This depends, of course, partly on his performance on PS and Cd.
From what you've reported, I wonder a little about the possibility of some additional speech and language concern, beyond articulation and rate, perhaps in the area of expressive language. Sometimes the rapid rate is a kind of masking device for underlying language issues. (The language on OLSAT is entirely receptive in nature, whereas expressive language is required on the WISC.) Has he been fully evaluated by a speech and language pathologist, not just for articulation reasons?
