The way I understand it, all kids will get RSV sooner or later. The idea of the synagis shots is to push it toward the "later" end of the timeline for kids at higher risk of suffering badly when they get RSV. Maybe your childs CF doctor could give you an opinion about whether Zeke seems strong/healthy enough to endure a bout of RSV this winter without any worse consequences than a nasty cold (which synagis won't do anything to prevent).