If the test was a cognitive abilities test, then some of the most important sections are designed to present problems which are novel to the test subject, who has to devise methods for solving them on the fly. How quickly and successfully they do this is an important measure of ability.
If the subject has already seen these types of questions and formed a method, they will perform faster next time. If they have had the opportunity to review their results and refine their methods, then they'll receive a boost to success rate. The results can no longer be used to compare against other test subjects who never saw those problems before.
This is why guidelines usually call for a full calendar year between assessments using the same instrument, or for choosing a different instrument using a completely different format if retesting needs to be completed sooner.
And now that they know your DS received prep for the first instrument, they'll be worried that he might have been prepped for their second one.
