My son had similar issues. Eighteen months ago his lung function dropped from 115% to around 65%. It stayed around 65-80% for well over a year. He felt great the whole time. His doctor tried everything and suspected ABPA. It wasn't until they did a second bronch that my son finally cultured it. After 3 months of treatment for ABPA his PFT's shot back up to 110% and have remained there the past few months. Hopefully, that is all behind us. Hope your son improves soon.
Sharon, mom of Sophia, 9 and Jack, 7 both with CF, Grant, 2 and Paige, 7 months both without CF