It seems to make sense that a child with an anxiety diagnosis might struggle with timed tests.
How long does it take him to do a similar test if you don't tell him it's timed? For my DD, until she could get used to it, we worked on timing how long it took to do the math fluency practice, instead of telling her it had to be in a given amount of time. That was enough release from the pressure to learn that she could actually do it.
Look around on the board for multiple threads on how math acceleration shouldn't hinge on doing the timed math tests.
I agree with polarbear that following up the evaluation with a request for a 504 for extended time or similar accommodation would be appropriate.
I can't figure out how to search this out on this forum or google for that matter. Any leads? Thanks!