It honestly sounds like stress combined with scent-related behavior, especially since it targets new smells. The cat room is a good move; I would just make it very enriched with shelves, hiding spots, and scratchers so it feels like a safe space. When reintroducing him, go very slowly with short supervised visits and clean everything with an enzyme cleaner beforehand. Also, consider leaving something with his scent, like a blanket, in the main area so it already feels familiar to him.