Pet passing away, how to tell child

Hey guys, my son is 2 and a half, and he has a rabbit at my moms house. He loves that rabbit and she is the first thing he goes to see when we get there. We recently found out she is sick and most likely won't make it.

My son remembers everything, so if we replace the rabbit, he will know. How can I tell him that the rabbit died? I know we can tell him she is living free and in the wild, but he is too young to know what that means.

I am at a loss, and just so sad to have to deal with this when he is so young and doesn't understand.