-Last night, my 12 year old stepson bit the head off of his pet rabbit?

Me and my husband are just so shocked and flabergasted right now. The head was on top of a pencil in his pencil holder cup, and the body was underneath his pillow. I guess he slept with it under his pillow. I don't even know what to say. I have to pick him up from school in a few hours, and I don't know how I am going to be able to sit in the same car as him without freaking out. He's hurt animals before, but he's never bitten off the heads before. As a step mom, I don't know how to talk to him without crossing the line, yet. I've only been his stepmom for two years, and we've never been too close. His father doesn't even know what to do. His mother lives in France and never returns our phone calls. What should we do?