How sad, poor little bun.
He could have died from anything, shock, heat, poison, etc

Such is the life of a wild rabbit, so not fair. Or maybe he lived and is back at being a wild bunny.
Here is a link on finding wildlife - http://www.wrn.bc.ca/found_wild_animal.htm
I have had many people bring me wildlife, mostly baby cottontail bunnies, we have a few wildlife rescue facilities in the area, it's always best to take them there if the animal is in distress. These people are trained to help the animal, and rehabilitate it as well.