Changing which Playstation account GTA Online is linked to
You can't migrate your GTA Online content between accounts. Your progress is permanently linked to your PSN account.
You can always try contacting Rockstar and see if they can help you. Access the GTA V Support Page, go to Online Play -> GamePlay Progress -> PS4 -> Contact Us
If they can't help you, I believe the best solution is to create a new account to play Call of Duty. This way your brother keeps his GTA Online progress and you both can play simultaneously on different devices.