Was it a CF doctor who told you to give your son mucinex? We were always advised to avoid OTC cold medicines such as mucinex, decongestants (oral or nasal).... When DS does get a cold, we have him do sinus rinses. His nasal mucus is so thick his nose never runs. Sometimes when he was younger we'd steam up the bathroom and have him play in the tub for awhile. He also does daily nebulizer treatments with albuterol and atrovent with chest physiotherapy (now uses the vest). Some people use nebulized hypertonic saline, some use mucomyst (sp) as well. DS also does a nebulizer treatment once a day of pulmozyme (DNase) which helps to thin mucus.