Re the math facts - at 8 years old, he might simply not be developmentally ready to memorize them, even if he's profoundly gifted. Math facts are conquered at different ages for different kids, and for some kids they don't come until as late as 4th/5th grade. However - it sounds like your ds does know his math facts because you can quiz him verbally and he knows them. So you have to look at what's the difference when he's tested at school - and I'm guessing the difference is he's using handwriting to answer on the timed tests. So chances are it's not knowledge of the facts that's holding him up, it's handwriting.
So what can you do about that, if it's handwriting? You can ask that he be tested by responding orally in the classroom. You might not get that unless your ds has an IEP or 504 with accommodations, but you can at least ask that it happen at least once so the teacher can see the difference in his scores.
The other thing you should do is to try to see if you see handwriting holding him up anywhere else in his academics. Are any of the other WJ-III frequency tests low scores relative to the others? Does your ds resist or procrastinate on other types of homework that require handwriting?
All of this may be totally irrelevant for your ds, so ignore it if if doesn't sound applicable! OTOH, if any of it sounds familiar, fwiw, my ds has a challenge with handwriting (dysgraphia), and none of us recognized it until he was evaluated by a neuropsych in 2nd grade. We didn't seek out the neuropsych because we were worried about handwriting, but instead we were referred to a neuropsych because our ds' teacher was convinced he had ADHD, and we were worried because he was displaying signs of sky-high anxiety. What we have learned since that time is that he doesn't have ADHD and although he is a person who reacts to stress with anxiety, when he is given accommodations for his challenges (handwriting and expressive language) the anxiety disappears - it was completely secondary to what the real issue was.
That doesn't mean your ds has a handwriting challenge, but I just mention it because a neuropsych eval was key for us in understanding what was really going on at school for our ds. Up until the point his anxiety started ramping up at school, all we saw as parents was our son, whom we knew was extremely bright from his verbal conversations at home, was being held back in math because he wasn't writing down 20 (or something) simple math facts correctly in 2 minutes. Which was of course was beyond puzzling to us since we knew he knew those facts... but we never bothered asking him to show us with handwriting, we were just asking him to answer verbally when we quizzed him. For everything else, he was compensating which he could do simply because he was so smart. Eventually the handwriting demands became so high in other areas that he wasn't able to compensate, and all of that started coming together in 2nd grade.
Best wishes,
polarbear
