[Engel]

Agree with the above, possibly cushings so will need vet attention to manage it properly.
How old is he?
Is there anything in his cage like a tube or jar that he could be scraping his back against?
Is the hair falling out in your hands? Like if you gently pull the hair does it just come out? or is it a gradual loss of hair?
Another possibility is mites which will need to be checked by vets or hes simply moulting.