Some people have a strong asthma component to their cf (not a separate condition per se, it's just one of the ways cf can impact the respiratory system.) Some cf centers/individual drs are much more informed about this than others. Fortunately, our clinic is one of the more informed ones, since Emily has significant issues with this.
The 'asthma' component in very simple terms comes down to problems with chronic inflammation in the airways. Like your son, Emily had a very nasty illness almost a yr ago requiring a few rounds of Prednisone to help her get over it when the abx just were not working. It was a different kind of cough than what we had ever seen before. After she responded well to that, but needed it again with her next illness we decided to start Pulmicort for the duration of the illness (a nebulized steroid) and then Flovent (by inhaler) for daily use. It's made a dramatic difference for her. Keeping the inflammation down has allowed her to recover from illness and congestion quicker since she can cough stuff up better.
What to expect: oral prednisone makes Emily cranky and once she starts feeling a little better, she can be quite the insomniac. These are very common side effects of prednisone. It can also cause increased appetite, which I consider to be a positive side effect given our kids can lose so much weight when they are sick. I assume Joshua will only be on prednisone for the short term. We have no experience with Xopenex but it's my understanding that it's a bronchodilator similar to albuteral, which Emily does fine with. Flovent Emily does fine with as well. In case they never told you- make sure that you have him rinse his mouth out well (and spit, not swallow- if possible... I know he's little) after he uses it, since oral steroids can cause thrush.
I hope these meds help Joshua... I can well imagine how overwhelming it would feel to add all that at once! Emily has 4 different inhaled meds but we fortunately added them one at a time so we were able to gradually get used to them.