Sync iPhone to iTunes for the first time after using it?

Help please. My dad has an iPhone 5 and has been using it several months but never synced it to any computer. Now I want to sync it to my MacBook to have it backed up. When I connect it, iTunes asks me if i want to set it up as a new iPhone or restore from a previous backup (i have backups for my iPad and iPhone). What should I do to back it up to my computer without deleting any data from his iPhone? Thanks in advance.