Hi Betty -- this question is almost a year old, but I'd still like to answer.

One way to do it is having your son sign in, go to whatever Mission he's working on, and clicking on "Practice".  This lists out all the skills for that Mission and the ones he's mastered will show up as blue stars (the ones he hasn't completed will be grey stars).  There will also be a green check mark next to any completed sections:


The other way is if he has added you as a Coach.  Then you can sign into your account, go to the Coach dashboard, (if you haven't created a class and added your son to that class, then do that), go to the class he's in, select the Mission he's working on and the time frame you're interested in (you could choose activity from all time if you wanted everything), then click on your son's name.  This will list all the skills he's attempted, the level he's at, how many problems he attempted and how much time he spent, on each skill:

Hopefully that gives you what you need -- if you have any questions, just reply here and I'll try to clarify.  :-)