Take a look at this article - you might want to print and share with him.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6688147/
A consult with your ID doc might be in order or a discussion with the pulmo about their objection.
Also, at this point in time, I think if a pulmonologist is still not on board with the benefits of 7% saline, they may not be really experienced in treating MAC, because it is now a solid recommendation by centers of excellence like National Jewish Health (NJH) and Mayo Clinics.
Sue