If I'm understanding you correctly, he's since paid off the cards, right? If so, I would just write a letter stating just what you said - that he was in school, that he had to cut back hours at work and was unable to make the payments but as soon as he was able to work full time again, he made sure those accounts were cleared. Also state that he has a fulltime job and has worked to make sure his accounts remained in good standing since.
I didn't have to do this but I think that just telling it like it was is your best option. Especially since his recent credit history is good.