It could be many things, if you have not checked in with a vet already I recommend doing so, if it’s to costly then I would look into symptoms. It could be as simple as hormones to a bad rash. I’m thinking it’s juvenile hair loss. Juvenile hairlessness is a condition where a young rabbit starts losing fur. It is a rare condition. The fur either becomes thin, or hair is lost all over the body, except the tail, the extremities of the limbs, the ears, the nasal region. Fur may start to grow back after a few weeks.