I just wanted to comment on some of the facts surrounding such understandably very emotional situations, but before I do so I wanted to point out that this does not mean that I do not understand, appreciate or empathise with the emotions, just that I am focusing on the facts.
In the UK there is no legal right to request treatment, neither by patients nor by families. Treatment is provided when medical professionals deem it to be appropriate. If CPR is considered by medical professionals to be futile there is no right by the patient or family to request it.
Competent adults may refuse any treatment for any reason, even if the result is severe harm/death.
One question here is whether this patient is competent. If he is competent then he should have been consulted about the DNR, although he would only have had the right to refuse resus not request it. If he is not competent his family do not have a legal right to decide on his behalf, his doctors would make a decision based on his best interests, which could include the decision that attempting to prolong his life via resus would be against his best interests (in practice many docs do discuss such decisions with families as it is good practice to have agreement on withdrawal/withholding of treatment but they are not obliged to).
However, if, in deciding what was in the patient's best interests, doctors took into account irrelevant or discriminatory factors then the decision was not justified. It all comes down to how we assess best interests. If his Down's syndrom is a legitimate medical reason that affects his quality of life then it is acceptable to take it into account. If it is not a factor affecting quality of life then they discriminated against him (I suspect the latter to be the case but it will be hard to prove in court).