He could try Cryptomator (https://cryptomator.org https://cryptomator.org) which has iOS and Android Apps and can sync over Dropbox. The same service is also a

He could try Cryptomator (https://cryptomator.org https://cryptomator.org) which has iOS and Android Apps and can sync over Dropbox. The same service is also available trough Cyberduck and Mountain Duck (https://cyberduck.io/cryptomator/ https://cyberduck.io/cryptomator/).