Hold him nose down and do percussions on his back. This will help to expectorate the mucous. Fresh Garlic is good, add some food processed kale to the mash along with some olive oil. Isn't mullein an expectorant? Just remember that just because something comes from herbal origins, doesn't mean that it is completely safe. Many of the drugs we use every day and know to be lethal is certain doses came from herbal origins (aspirin, penicillin, silymarin, THC, etc...). Sometimes it is okay to give an animal antibiotics if it will spare the kidneys an onslaught of random chemicals. This is a very little chick whose kidneys aren't even fully functional. I would give the greens, garlic and olive oil as well as an antibiotic. Antibiotics are a focused attack on the pathogen and they save the kidneys from being overworked. It is okay to give antibiotics when they may mean the difference between life and death. It is not okay to give antibiotics on a prophylactic basis.