Nagging, tickling night-time cough

My husband had a cold recently. Almost 2 weeks ago, most of his symptoms went away, but he developed an annoying, tickling night-time cough. He's been taking Robitussin at night, so he won't keep himself (or me) up all night. It works, but I'm not sure if it's good that he continue to take this.
Which Schulze formula should I order for him? I had read something about a sinus and lung formula, but can't find it on the website.
He has no other symptoms besides the cough (which usually begins around 8 or 9 pm). It's a dry cough, too, no phlegm or sputum.
Any suggestions would be greatly appreciated.