In regards to the yeast problem I would next time take my son off Gsh or just lower the dose to real small when he goes on Antibiotics again.
I for the first time noticed major thrush out his penis and all through his undies for a couple days. I backed right off the gsh as he is on Cirpoflox (antibiotic) and it went away. I will after he has finished the course in a week do a major push for garlic and probiotics.
Re other suuplements you need to introduce them slowly, only start one at a time and wait for 2-4 weeks before introducing another, someimes even longer depending on what it is.
Eg:Essential fatty acids(fish oil) you can add right away and you prob only need to wait a week, just make sure u give adequate enzymes with it as its oil. This is an antinflammatory and CF is known to be low in DHA.
I am not sure that you want to completly take away the oral gsh but to be mindful that yeast is a major prob for CF and to keep a close eye on it. Eg, itchy skin, thrush, malabsortion, headaches, excess thirst, sugar cravings, bloating, tummy pain. These can also be signs of other things too.
I would be esp aware any time he is on antibioics, that is just my take on it.
As for other supplments this is something you need to decide what you use and research what to give. Not everything that works well for one person works well for another, CF is very individual.
I give my son
NAC
VIT E
Probiotics
Good quality Vitamin/mineral
EPA/DHA ( Fish oil)
Magnesium (this is really important, lots of medicines deplete this)
GSH oral and neb
Hypertonic saline
Nose rinses
Feel free to ask questions

Monique