The file is corrupted. You wrote that you are working on a Mac. If you have Time Machine in and running, you may be able to recover an earlier version of your work.
If not, try to open the file in your Browser or other application. If that works, resave the file that appears. If not, your son may have had his first valuable lesson and begin again. In the future, he should save the file now and then while he is working on it, being sure to save it each time with a different name, such as: james_1, James_2, James_3, etc. That way, if the current file becomes corrupted, he has an earlier version to reach for.