I have to agree that the tapping on his chest and back can really help to loosen up his phlegm. This treatment is used in hospitals by nurses and doctor to help loosen the mucuous that is stuck in patients lungs and airways.
However, I have a treatment that is extremely effective. In Europe and other countries a nebulizer is commonly used for infants and young children to treat coughs and chest congestion. When we were overseas my son was seen by two doctors who both prescribed a nebulizer. The treatment is safe and makes a dramatic difference. Last year both of my sons had terrible coughs and chest congestion that prevented them from being able to sleep well and it really started to take a toll on them.
We visited their pediatrician and I asked her if she could prescribe a nebulizer and the solution that goes with it. She was willing to give it a try and I am so glad that she did. We picked up the prescription for the saline and medicine drops at the pharmacy (which are completely safe for babies and young children), and the prescription for a nebulizer was calling in from their doctor's office through a medical supply company and it was delivered in person the same day to my home. And insurance covered it.
For a safe, extremely effective treatment, I strongly encourage you to ask your pediatrician if he is willing to let you give it a try since your son has been so miserable. Plus, once you have the nebulizer, you can use it in the future when he gets a bad cough or congestion. Basically, you just use the little mask that is connected through tubes to the nebulizer, and your son breathes in the mist and usually within 5-10 he can breathe much easier and the cough settles.
Good luck!