Did you try Photos 1.1 from El Cap? I didn't upgrade yet, but it seems that it supports sorting:
Photos has been fine-tuned to make it even easier to manage your library. Now you can add a location to a single image or to an entire Moment. Naming your favorite people in Faces is faster with a streamlined workflow. You can also sort your albums ? and the contents inside them ? by date, title, and more.