Breathing Problems

I am an Age Group and Masters Swim Coach While I am very comfortable working with strong swimmers, sometimes I teach lessons to adults who want to do triathlons and are fairly new to swimming. Every so often, I find a swimmer who has problems breathing. I have already read previous discussions where breathing is a problem and all the suggestions recommend improving stroke technique. I am very aware of the benefits of TI and like to encorporate it whenever I can. I have a very fit runner who now has pretty good stroke technique but after 25 yards is too out of breath to continue. Being a life-long swimmer, and having 8 year old swimmers who can go 1000 yards without problems, I can't fathom how anyone can't "breathe". This sounds very basic but in order to get a good diagnosis, I will try to be very specific. He is exhaling slowly, continuously, and completely through his nose before he rolls to inhale through his mouth. I generally have him breath every 3 strokes, but sometimes vary it and nothing seems to help. Has anyone encountered this and if so, are there better drills than just simple bobs?