My nephew is perfectly healthy (20months) but 3x now he has gotten a cold and has needed puffers and all sorts of meds because he quickly develops pneumonia.

He did get a chest xray, which should have ruled out this though. But perhaps it has progressed. I would film it. At pick up pull them aside and say you are concerned, you filmed one of his episodes so they can see what you are observing, he may be more active here so they are not seeing the same thing. Do it in a concerned, proactive manner not a "I told you he has a problem" kinda tone so they don't feel like you think they are sucking as parents! There is a good chance at home he is on the couch watching tv so isn't as bad.