From your description he is still on the antibiotic amoxicillin. You need to be sure to continue that for at least 5 days after he seems fine. So far as cough syrups go I prefer to use an expectorant to help the dog cough rather than a suppressant to try to cover the cough. A cough is a natural reflex attempting to free up fluid or debris in the trachea or pharynx. Ask your pharmacist to point you to straight guaifenesin syrup and give 0.2cc per pound of body weight 2-3 times daily by mouth. Take a look at the Respiratory Disease videos for additional information.