This comment has been removed by the author.
Let me try that again: I know this is going to sound weird, but: Rub Vick's VapoRub into the bottom of his feet.
Delsym (http://www.delsym.com/) is the only thing we've found works for coughs. I end up taking it in the fall for those allergies. Though it's all about runny eyes in the spring.
If he feels like he's trying to clear nasal/sinus congestion, try nasal irrigation. Either a neti pot or get a few bottles of saline nasal spray (the store brand is about $2 for a small bottle) and have him squirt it up there to loosen up the congestion. Once it's moistened (and yeah, this is gross), it'll blow out a little easier.If it's a throat-based cough, I find hard candy works, although sugar free hard candy might not work as well (and you need to be careful about dosage and side effects with sorbitol), or hot lemonade with honey (I need the sour to counteract the sweetness of the honey - any hot beverage will do), but I'm not sure if he can do that.