There’s no—or Infinitesimal— difference between four classes totaling 15 credits and four classes totaling 16 credits. A “credit” just means the number of hours it meets per week, not necessarily the work load. It’s an old designation that seems to have stuck throughout the years. Indeed, I would argue that five classes at three credits each, for a total of 15 credits, is a much bigger load than the 16-credit load he has.
Is he a first-generation student?