It sounds like the python still has a respiratory infection. Drooling is one of the symptoms and the discharge can be many different colors (or it could be that it is clear, but sticky and is being stained by the substrate you are using).
Either the initial round of antibiotics wasn't given for long enough duration to kill off all of the bacteria or it is also possible that he had more than one type of bacteria present and the antibiotic was not the proper treatment for the secondary infection.
I'm sure you already tried some of the standard "at home" techniques (such as raising the temperature and adjusting the humidity). Be sure that you have also sterilized his environment during the course of his treatment.
Remove all cage furniture (water dishes, hides, etc.) and soak them in a bleach and water solution according the directions on the back of your bleach bottle (not all bleach is of the same strength/concentration so it is best to read the bottle).
Place the snake in a safe secure container (a sealed rubbermaid works well) and wipe down the interior of his container with your bleach and water solution. Allow everything to air dry.
I usually follow disinfection with an actual cleaning. Cleaning and disinfecting are not the same thing. Simple soap and water will work for this process. The bleach is used to kill harmful bacteria while the soap is needed to break the surface tension of the water and allow you to actually remove the dirt and debris (even that stuff you can't see).
Again, let everything dry thoroughly.
During the course of treating his RI don't use your regular substrate. Instead, use newspaper, or some sort of paper "liner". This will allow you to see any discharge much easier as well as making it easier to clean. Plus, you won't have to worry about any substrate sticking to (and possibly blocking) his mouth or nostrils.
I would suggest bringing the snake back to the vet (call around and find a vet that has experience with reptiles). Let us know where you are located and maybe someone on here lives near you and can either check out your setup or suggest a good vet in your area (or both).
The forums are pretty good for general care, but at the end of the day, if your snake is sick or injured there isn't much we can do (even if our diagnosis is correct). Yes, a vet can get to be expensive, but if you can find a good one and get this illness cleared up it can be worth it in the end. Remember, these animals can live 30 years with proper care and in that light the cost of a vet over 30 years is actually less than it would be for a dog or cat.
Once this gets cleared up, if you and your son can maintain the proper temperatures, humidity and other cage requirements you two should have a pet that rarely (if ever) gets sick again. I've only dealt with one case of RI (a little over ten years ago when I first started keeping herps) and since then every animal has been healthy.
-----
.
.
.
.
Stay United!

"...I say 'apparently' because despite all our progress she is not at all a tame or handle-able snake and gettting her from that cage would re-start a war.. and we've had a good armistice for several months now."-Gus Rentfro - I love this quote!