The entire phrase is "fat, happy wheezer" and it's one of those cute medical terms that is falling out of fashion. It refers to a child that has an ongoing mild bronchospasm (narrowing of the airways) during infancy, but has no other symptoms or ill effects as a result. It is similar, and sometimes used to refer to, post-infection airway hypersensitivity, which is where a cough or wheeze persists for weeks or months after an infection, even though the other symptoms (fevers, or difficulty feeding or breathing) have gone. If you compare it to the alternative, which is an acutely unwell child, it's actually a good thing You are right, though, in that he may be prone to asthma later in life, and some "happy wheezers" do get sicker, quicker, when they get upper respiratory tract infections.