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His reading comprehension level per the CAT test when he was 6 years old was 5th grade, and he reads books on about a 5th-6th grade level now and understands what he reads.

I'm no expert, but his verbal scores seem surprisingly low if this is the case. The above sentence approximately describes my DD's verbal capacities at your son's age and her verbal score was in the high 130s. He certainly sounds verbally gifted to some degree or another if he was comprehending 5th grade books at age 6. Could there be another issue at play here?