I am very sorry to hear that your son is so sick. I am concerned about them putting him on a clinical trial BEFORE using 5-FU and Cisplatin.
It does seem from what you are telling us that your son is getting very sick, very fast. Does he have a feeding tube, and where is he being treated?
This disease is horrible, I watched my father deteriorate in a short 7 months. I cannot tell you what to expect as everyones situation is different. If they are giving him a prognosis of 6 months, then it does not sound like they are hopeful, and it could be longer than that, or shorter. Does he have mets? Where has it spread?