Hello, Thank you for asking at HCM. I went through your history and would like to make suggestions for your son as follows: 1. Generally speaking, there are three main causes of difficulty breathing in a child with bad cold & cough at this age: obstructed nose, wheezing (airway constriction), progression to pneumonia - treatment of both of them is entirely different and only a doctor can identify the cause by examining the child. 2. Of course, Mucinex cannot cause such symptoms, so please continue the medicines prescribed by the doctor. But were I treating your son, I would suggest him to get examined by the pediatrician so he can identify the cause of breathing difficulty and treat accordingly. It may be simply a more severe cough-cold or may be progression of the disorder/infection itself to airways/lungs. 3. As the treatment of all three possibility is very different, I would suggest you to get your son examined by nearby pediatrician. Hope above suggestions will be helpful to you. Should you have any further query, please feel free to ask at HCM. Wish your son the best of the health ahead and a very good recovery. Thank you & Regards.