M.B.
J.,
I would sit down with him at some point when he's calm, not freaked out, and talk with him about it all. Explain to him that Auntie and Uncle were very very sick, and that it was their time to go to heaven. Also explain to him that Gramma and Grampa aren't going anywhere any time soon, that you'll be around for a long time (hopefully). Answer as honestly and truthfully as you possibly can, kids know when they're lied to.
Recently I have discovered the truth of the saying "kids will ask the questions when they can handle/deal with the answers". Of all the school shootings on the news lately, my 4 1/2 year old son has only asked questions about the NIU one.
Hope this helps, I don't know what else to suggest,
Melissa