I think this is a decision made by his parents, or those who replace them.
I think the responsibility is too serious for anyone else to decide.
How much is the disease deadly? What is the actual forecast, what are the options? How reliable and definitive is the diagnosis? - These are not empty questions.
If there is a decision to speak, then - with the participation of a professional psychologist. And the moment, and after. Because to say and quit, to throw alone to be left with this knowledge is something about stupidity or sadism. Those who do not undertake to provide full and sufficient support and support to the very end have no right at all, anything is "decided" here: not their business.