You have got to try the Vicks vapor rub on his feet at night.... please read my story it's a little long but i'm very familiar with the dry cough i dealt with it for 7 years before finding an answer...
My son has had allergies from the time he was 3. It always started with a cold, we actually ended up in the hospital due to an asthma attack at one point, but it always seemed to be triggered by a cold... we were not told right away to go to the allergist and being a young mother with no friends with children I didn't know to ask. My son had the worst dry cough at times and it always happened after a cold was pretty much gone. Night time was the worst. I went through so many sleepless nights and the doctors would look at me like I was nuts when I told them the cough meds don't work for him with this cough. (I know now from news reports that I’m not crazy) but anyway, when he was 5, finally a doctor told me to go to an allergist and we found out what he was allergic to and I did my best to keep him away from that stuff so he wouldn't go into these episodes in the first place. (It was cats and dust and basically indoor allergies so it's not easy, especially in the winter) the allergist explained to me that the allergens started the asthma process not the cold. We still had these coughing episodes no matter what they prescribed to him always after a head cold – by the way, a doctor said the word habit to me once, and I looked at him like he was crazy this time – even tho it all just didn't seem right, I just figured this is something that we were going to have to live with - after 7 years of dealing I guess I just started to think, ok this is just how it is when he gets sick or is around the allergens... Anyway, Last year when he was 10, I heard about the Vicks on the feet... thinking it was ridiculas, but after 3 nights of little sleep from a dry cough (that came on him after he was sick), with lack of a better idea I tried it... guess what??? It worked and I was totally impressed. I’m not kidding when I tell you, it was like magic. Now... one thing I will say, when he gets a cough due to cold (the raspy cough), it does not work, but for the asthma type dry cough that you seem to be describing (and you say it is after a cold, which is the same time my son would get this dry cough)... it works for him. so, it's not going to cost more than $5 to try it, I wish someone would've told me when my son was 5, there would've been much more sleep in our lives :) It’s amazing that I have gone from just dealing with this situation to not worrying about it anymore – such a relief! Good luck to you; I can relate to what you are going through!!!!
S.