One way that you may consider helping your son out is this: make sure that the air that he breathes is as clean as possible. This means NOT living the typical Amercian lifestyle with air fresheners in every room, and anti-wrinkle and anti-static products sprayed. Use the most natural soaps and laundry products which might mean going to a health food store and paying a little more. Don't be heavily perfumed yourself,etc.
I do not offer this advice as a cure. But with all of your child's problems, it is important that you maximize the things that you can do in the environment to make sure that the air he (or she) breathes is as clean as possible. (You'll be healthier for it yourself)
This is obviously a very difficult problem for your son and for you. There are several reasons for your son to have this difficult time with coughing. It is essential for someone to determine the reason for the cough in order for a specific approach to its treatment to be developed. Because of all the problems your baby had during the early months of his life, I would highly recommend that you take your son to a Pediatric Pulmonologist to evaluate the cause of the cough. This could be an airway problem. This could be a lung tissue problem. This could be a reflux problem. Someone has to help you by finding the cause. I would also recommend never giving your son "some granules" unless you knew what they were, what they were supposed to do and had some written information about them.
Good luck