You can find DIEBELS online for free:
https://dibels.uoregon.edu/market/assessment/material/If you follow the link and go to the 2nd grade, you will be able to download the entire test booklet including the script.
Scores like your DS indicate to me many possible reasons for the discrepency. It could be lack of exposure to phonics instruction which does not seem to be the case for him. Or, it could be stealth dyslexia. If he is using memorization as him primarily strategy, he *could* start to struggle in 3rd grade when reading materials would include more unfamiliar words. Another possibility is that it was a task that made him rather anxious and he froze up.
I wouldn't worry about being able to read nonsense words per se but I would try to see how he copes with non-fiction materials outside of his comfort zone.
