Be really careful about raw honey even mixed into milk. Children under 3 yrs old have digestive systems that aren't as developed to handle the botulism spores that are in raw honey. Obviously raw honey on a spoon is a choking hazard and should never be given.
The directions on the containers of medication you bought ought to have dosages and the time frames for those dosages ie. when the doses would wear off. I would think that after the CF wears off per the directions he'll be perfectly fine to have the chestal.
It's difficult with cough medications though, because most won't recommend giving them to a child that's under 3 years old. There's a Vick's vaporub for children that my kids love, as well as hot showers. I don't offer them anything with dairy at all when they have a cough because dairy produces phlegm and will make them cough more in the long run.