Jim, I'm so sorry to hear this. This is exactly how a coworker passed away a few years back, and it's hard because you don't ever get to prepare, to say goodbye, to spend those last precious moments. And if it's that hard with a coworker, it's infinitely impossibly harder with a parent.
I'm no stranger to loss and can say that each day will bring change, for the worse as shock wears off, but for the better. Cherish his memory, and thanks for putting the love out here for us to see. You know you'll get love back.