Oh, lovely, I wish I wasn't reading this
Play it straight with your boy and answer his questions that he'll ask. I'm answering 3.5yo questions about a family friend who passed away and I answer him. His little brain makes other things of it, but he'll get it one day (like just now he's saying that our friend died and he will be magicked back and he won't be sick anymore, so I just say "I hope so"!).
All you can do is be there for him and if he's ok with it for now, then he's ok with it for now and you can get back to you
I'm so sorry that the pg wasn't to be