With whatever form you take with his body, a simple ceremony of asking for peace from the directions, and then for him to be guided safely into the Summerlands (or your preferred name for the afterlife) without fear. You can ask that he feels the love you (and others) give him, and tell his spirit that he'll be reunited to his rabbit oversoul. Ask the God/dess to bless him and look out for this little one. Bless his body with the elements - water & incense , and the earth where he lies/is buried.
Or any version of the above that comes to mind.
Coifi was a long time member of OBOD, now deceased. He shared his prayer for road kill animals - I say it whenever I see an animal by the side of the road.