What Causes Rashes All Over The Body?

My 15 year old son has had a series of illnesses that may or may not be related. He weights 95lbs, stands 5 5 and has always been very skinny and one of the smaller kids in his class. He is very smart, enjoys school, is in honors bio, honors English, and honors algebra and is taking Latin as his elective language. Three weeks ago he had a very mild fever for only one day, but was very fatigued and had a nagging deep cough. We took him to the doctor who gave him a course of Z pack Which seamed to work. So we sent him back to school after two days. But he continued to be fatigued very easily and the cough persisted. He has a history of persistent coughs that tend to last for weeks at a time, so I wasn t as worried about the persistence of the cough. But I was worried about the sound of the cough. It was a very deep cough, not a scratchy throat kind of cough. Last week he was so fatigued and just constantly exhausted that he couldn t function normally. So we took him back to the doctor again (on Monday, Feb 12). She prescribed 875 MG of Amox-Clav to be taken twice a day. And sent us to a lab to have a bunch of blood tests. I don t know which ones but nothing came back negative. Nothing seamed to work. He was just out of it for a week. He slept all night and all day. He would wake up to eat (he retained his appetite) and use the toilet. I would wake him up a few times each day to see if he would try to set up and watch a TV show with me. But after 10 to 15 minutes he was out of it and went back to bed. He was out of school the entire week. On Wednesday the 14th he developed sever diarrhea and lost his appetite. On Thursday the 15th we scheduled another appointment with doctor for the next morning. As luck would have it, he was actually doing better on Friday the 16th. Diarrhea was diminished, he was alert, no fever, so sore throat (only a mild nagging throaty cough he tends to get sometimes anyway). He was ready to go back to school except the school was closed for in-service and parent/teacher conferences. We took him to the Dr anyway and then to get some more bloodwork. We were thinking that it could be Mono. But we seamed to be past it, what it was. He spent the weekend at his mothers house and she called me on Monday the 19th to tell me his body is covered with a rash that looks like chicken pox, except he has been immunized so it is highly unlikely to be that. Other than the rash, he feels fine and ready to return to school So she calls the Doctor who tells her over the phone that it is Hand Foot and Mouth Disease and that he probably caught the Coxsackie virus. I was concerned about the way this was diagnosed. I explained to her that I m allergic to Amoxicillin and I know that can cause a rash. And since this is the first time our son has been prescribed Amoxicillin it is possible that he is also allergic. So she called the doctor back the next day and asked about that. Apparently the Doctors said that because the rash was also on his hands and feet that it couldn t be an allergic reaction. I picked him up from his mothers today (2/21) so this is first I ve seen the rash. His Torso is covered front and back and it has extended down his arms and legs to the tops of his hands and feet. But nothing on the palms of the hands, bottom of the foot, or around the mouth. I couldn t see anything on his face. It does not look like chicken pox. There are no pustules, no liquid pockets of any type. Just tiny raised bumps. No large bumps. There isn t much discoloration. He reports no itching of any kind. He has no fever, has a good appetite, no intestinal issues. No other symptoms of any kind (other than that nagging cough I mentioned earlier). The tiny bumps of the rash has a greater resemblance to the measles than pox. But he s had all his shots so it can t be that either. The rash has some redness, but not a lot. It s not a bright red. Rash appears to be all small bumps, no large ones, no merging into giant rash areas. There doesn t appear to be one area that has a greater concentration than any other. I don t like contradicting a doctor, but I really don t think this is Hand Foot and Mouth Disease. It could still be a result of one of the Coxsackie strains, but I ve spent several hours this evening looking through all the diagnostic criteria and thousands of photos of rashes attributed to Coxsackie and my Son s symptoms and rash don t appear to match any of the criteria. Of course I understand the Coxsackie can present differently and that a person could get any one of the symptoms without any of the others. And since most Coxsackie diagnosis are from observation and reporting rather than clinical testing, I guess a doctor can just attribute anything they don t understand to Coxsackie, especially if it is know that there are other cases in the area. But my question is that if it s not Coxsackie, then what?