He knows the math facts. He simply cannot produce them when a timer is involved.
That, coupled with the other behaviors you mentioned, leads me to wonder if anxiety might not be worth an additional look.
ETA: How does he do if YOU time him-- on the sly? That is, if he doesn't know that he's being timed, how does he do?
Pretty much the same. The reason I thought ADHD is that he would stare at a single problem for up to 30 seconds, and he sometimes couldn't sit still while doing practice tests. And in many cases it's a problem he has already correctly answered earlier on the same page.
What changes if you then let him know that you're tracking how long it takes him?
What is different if he is running OUT of time, instead?
If it's anxiety driving things, those three situations should result in progressively worse performance.
We have tried each of those scenarios, and nothing changed. When we simply had him do all 100 problems in whatever time it took him, he usually took 13 - 14 minutes to complete.
Now there IS recent progress, in that he now confidently skips a problem he can't recall the answer for immediately, and comes back to it later. He is successfully completing the 100 questions in 7 minutes or so. The first time he did this was last week, and he passed his -4 on first try. While I was writing this post, he was working on his -5 for the first time. He got all 100 done in 7.5 minutes.
Perfectionists are very prone to anxiety, and your last statement above is a classic 'tell' for perfectionism (the outcome is no longer certain, this feels hard-- time to quit while I'm ahead rather than risk failing).
I can see how it may be some perfectionism causing some of his issues, but I haven't really observed anxiety. I also wonder if maybe it doesn't keep his focus because of the monotony of it. Does that make any sense at all?
