Hi discodad, and welcome!
I'm afraid you are on a hiding to nothing with this one, and CEM is absolutely intransigent on the matter.
There are exemptions under the DPA for certain information relating to examinations, and the one CEM is using to withhold the information appears to be this (from Page 53, here: https://ico.org.uk/media/for-organisati ... actice.pdf
Information comprising the answers given by a candidate during an examination are exempt from the right of subject access. So a SAR cannot be used to obtain a copy of an individual’s examination script.
You could try challenging that through the Information Commissioner's Office, although it could be rather a lot of trouble for something that still might not prove what you are looking for.
I would advise you instead to focus your efforts on proving that the result (overall and in each of the three aspects of the test) did not reflect your son's ability.