Hi there Claire - interstitial lung disease isn't something we hear of too often on forum, although of course it is out there. My father developed a form of interstitial lung disease, related to his rheumatoid arthritis, then became diabetic, as a result of the treatment he was on at the time.
I'm sure the members here will support you as well as possible, but for more specific information, relating to his lungs, a specials lung/chest forum could also be helpful. Interstitial lung disease in a bit of an umbrella name for a handful of conditions, with a good number of similarities. On that basis, your brother's specific treatment and progress would depend on the underlying cause.
Have you looked for a lung/chest focused forum for information? I wonder if this group could be helpful to you? https://healthunlocked.com/blf/about
I am a member of Health Unlocked for my thyroid, but I haven't any personal experience of that forum, so I can offer you any wisdom on what it's like.