I don't know how old your son is, but those are the kinds of scores that may be associated with a strong (i.e. genius level) math ability.
We've just discovered through a variety of language tests (screened for 2e) that a gifted working memory doesn't translate to strong comprehension. My DS didn't hit the ceiling but he got an 18 in letter/number sequencing, and his language tests put him at the ceiling (19) for auditory recall.
Comprehension was good on the WISC (95th percentile) and much lower on more sensitive language testing.
Hmmm...and granted I may very well be projecting my own situation, but my DS was just diagnosed with an expressive/receptive language impairment. Has your DS had a thorough speech screening? I think this aspect of development was overlooked in my son because he has some extreme strengths. We're having a lot of lightbulb moments around here, as it becomes clear that he is 2e with average to low average social and abstract communication.
For what it's worth, my DS's PRI wasn't as strong as your DS's but still gifted. He far prefers math, and discrete challenging (non-wordy), tasks. Will solve math problems without batting an eye, years over grade level. Will have a toddler tantrum over age appropriate language homework.
