I'm sure my DS will pass after all the education and prep he will receive, but knowing that for other professional exams such as the Law Bar and Accounting CPA it is not unusual to have to retake it or at least retake sections of the exam and that seems to be the case with the CG exam based on what I am reading on this post from others.
You're making a lot of bad assumptions here. . .You cant equate the maritime license to a CPA or Bar, which are professional exams for people with usually graduate level education, depending on state requirements for the CPA. The engine license exam is only 70% pass for everything and most of them finished the modules in less than an hour. Most people say deck license is a little harder I think, but nowhere approaching a bar or CPA exam. Thinking about your son's performance on the CG license at this point doesn't really make sense.