I'd suggest starting here: https://support.khanacademy.org/hc/en-us/articles/204784264-I-m-a-teacher-coach-or-parent-how-do-I-start-using-the-site- 

Set up your account as his parent or teacher.  From there, viewing his progress will be much easier!  Use Progress to see the overall picture, Activity Overview->skills to narrow in on specific skills within a course and section.  You can even assign specific lessons for him to do, to work on specific areas.