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Did anything change before he began behaving like this, big cage clean, anything new in or around his cage, anything that could possibly have upset him? Sometimes it's not always easy to know if something has spooked them so it can take a bit of thinking about.
Even using different soap or something can sometimes really put them off!
Is he eating & drinking ok as far as you can tell? It does sound as though he may be unwell, they are very good at hiding signs of illness or pain, so check how he's eating & if it goes on for a while then try to get him out for another health check.