Oh my gosh. I'm so sorry. I know how you feel. After Chico and Chica were born I went out to their mother's rabbit house to check up on them and their mom and their mom was dead and two of the kits. Kits I could see dying, their mom, NO. Same thing. I was a horrible wreak and to this day I still have no idea what the hell happened. If something tried to get into the cage and their died of heart attacks? I just don't know and to this day it still bothers me greatly.
I know it's going to be tough explaining and possibly you can also contact your vet and see what they say as well concerning the death to make certain none of your other bunnies might be infected with anything. That would be my top priority keeping the remaining healthy.
My heart goes out to you and I hope he is able to handle the loss well. None of us ever do. I mean, we know at some point our bunnies, ourselves, everyone is going to pass but when it does it always comes too soon. And in this case, way too soon.
Take care
Vanessa