Well it sounds as though we might have the same then. It is called a specific antibody deficiency - a type of primary immunodeficiency. If you go to the messageboard of the Immunodeficiency Association, there are people there who might have more ideas.
As I said, the infusions are very expensive and do carry risks so they are not frequently prescribed, particularly for our kind of immunodeficiency which is generally not particularly problematic, apart from for a few individuals.
With physio, the huffing is hard, particularly when you're not well at the time. There are various 'devices' to make physio easier. Again, the physio can talk about these. I use the 'flutter' device, but there is also a thing called the 'a capella' or the pep. There is information on various cystic fibrosis websites (the condition is quite similar) if you google.
The symptoms you describe sound very much like a chronic infection, but I suppose the problem is that perhaps up til now they haven't worked out what. It could well be the MAC, but there could also be something else hiding behind that. Has he had a bronchoscopy to find out if there are any other bugs lurking down there?