My Son Has A Cough. I Gave Him Mucinex And

My son has a cough. I gave him Mucinex and Robitussin dm and he's still coughing. Trying to figure out if I can give him something that will help him sleep. I used to give him benadryl to stop the post nasal drip at night but I gave him the other two a few hours ago and I think it's too much "stuff".