My son is now 6 and a half months old and his weight was steadily increasing until about 6 weeks ago. We all had a really nasty cold/cough that for my 3 year old lasted a good 6 weeks and 3 weeks for my husband and I. However although my youngest was the last to get it he's still got his

(this is his first real cold).
He has been exclusively breast fed and we started weaning him onto solids when he was 6 months old. We've visited the doctor 3 times and been told that there is no infection (so no antibiotics given -

) but he just can't shift his cough. His cough is worse at night and sometimes he brings up part of his feed with his coughing. He is fine in himself (which the doctor said was the most important thing) but he hasn't put any weight on in 6 weeks and has gone down a 25%th percentile. As he suffers from mild eczema then the doctor has suggested that he could have asthma - so to go back to see him if he has his cough in 3 to 4 weeks.
I know I shouldn't get obsessed with weight gain but is there anything I can do to help him and his cold? I so want him to put weight on again.