Oh, OP,
So very sorry to hear. Unexpected death, in those circumstances, makes an already hard situation that much more so. 
He was indeed a beautiful cat.
How old is your DS?
Depending on age, I'd keep the explanation brief and factual, and stress that it's very normal to feel sad/ shocked/ angry/ upset that he never got to say goodbye properly. That you do, too, and that shows how much your beautiful cat was loved and cared for, and part of the family.
(And that his brother will be feeling confused, too, so in need of extra TLC) 
Perhaps together choose a lovely (container?) plant in his memory, as a place where your DS can feel close to him (ashes in compost, maybe?)...he could help make a little name/ date slate.
Scrapbook of special, happy, downright funny or odd memories of what your cat got up to? Drawing/ poem/ framed photo for his bedroom?
Never easy, with kids and pet attachments, is it?
Best of luck x
Also, for you, this book may already be well-known to many MNers; never fails me as a comforting re-read after losing a beloved animal companion...