I think you hit the nail on the head when you mentioned that between appointments, it is very difficult to know what bugs you may have contracted. Not only that, but quarterly cultures, in my oppinion, are not the least bit conclusive as to what is truly growing in the lungs. The sample could have been coughed up from any one of the many airways in the lungs, so in my mind, so much can be missed.
The one thing you have going for you is that Phillip is still young and in a stroller. You can very easily keep others from getting near him, touching him, breathing on him...
However, precautions at CF events are not taken seriously at all. Cfers should not touch anything that another cfer touches. Yet, Great Strides sites often have buffet lunches after the walk, silent auctions have pens that are picked up and set back down by many guests, etc. I worked for the CFF at the time that they started requiring chapters to include the B.Cepacia blurb on all of their invites. Verbage on invites is not enough. Most chapters miss some of the most obvious means of cross-infection at events.
Others may disagree, but I think Phillip is okay this year. My son will probably walk with us, but you won't find any of us mingling afterward, and certainly not standing in a buffet line for lunch. I know it stinks that we have to do this, but I would rather be safe than sorry.