If your son were truly in such a low percentile, you would have known he had a problem before now.
Things can happen during the testing situation itself, causing scores to be skewed. Sometimes it can be as simple as the student somehow getting the lines on the answer sheet incorrectly matched to the question numbers.
Also, sometimes teenagers do not take those tests seriously (imagine that) and just take a random approach to coloring in those circles. It even can be a planned move. -- My daughter, who is now a responsible adult with two degrees, told me that when she was a sophomore, she and her friends shifted answer sheets for one of the sections. (They don't call them sophomoric for nothing.) -- Now, please do not get upset, thinking that I am saying your kid would flagrantly mess with the test. I am just telling you that mine did.
I am very familiar with a situation when standardized state tests were incorrectly scored on a large scale. Aware teachers knew the scores had to be wrong. It was an across-the-board mistake. Caught and corrected. (That is not the norm. But it did happen.)
I am not trying to minimize your concern. I am simply sharing situations I have known.
Now, back to the concern about his not being a reader. . .
If I were you, I would find out if your local library has a Teen Librarian. . .
Teen Librarians specialize in what the library biz calls YA books. (Young Adult) Parents are often familiar with Children's Librarians but might not know that there are also Teen Librarians. Find one.
Teen Librarians (sometimes called YA Librarians) know what teens are reading now. The librarian should have a little conversation to try to establish interests and then will find some titles that might be a fit. Take home more than one so there are choices. Maybe you could read the same ones so the two of you can discuss the books a bit.
Do not torment him with required reading lists. Help him find books he will like reading. And you can do that by finding a librarian who knows the territory. Good librarians stay current. And I think you will be surprised at the number of books out there for teens -- and you will like them, too, I bet.
And, yes, I just might know what I am talking about. Now, get thee to a library.
-- It might turn out to be a really good low-key way to show your son the joy of reading for fun. And with that will come comprehension and concentration skills that can translate to his studies.