Airway clearance is something that should be done daily, and the vest is an extremely valuable tool in accomplishing this.
For kids, generally speaking, it's not a matter of waiting for declining health to get it but rather when they grow big enough for it, it is ordered and started.
Preventative care helps KEEP the airways clear, keep mucus from building up, and reduce illness/infection- and it's impossible to do that if it's only done when sick. Given he has clear resp involvement (asthma, pft's what they are), standard care for cf would be daily treatments, and increased treatments when ill. Being as proactive as possible will help boost his odds as much as possible for his future health... The vest might help increase odds of compliance there, since it allows for complete independence w/ treatments (especially valuable with that age.)
I wish the best and hope that he's able to get one.