Dgestive Problems that have gone undiagnosed for years!!!

I am a mom that has come to the end of my rope!!! My 9 year old son has dealt with digestive problems since he was eighteen months old. It began with him vomiting in the night that coincided with severe diarrhea. The first "episode went on for seven or eight nights… always throwing up in the night, the diarrhea would last for the duration. At first we chalked it up to a really bad case of the flu. But after a week's reprive, it started up again.. vomiting in the night and severe diarrhea. Something that struck us as odd was that his vomit was always brown and the smell of it was enough to drive you out of the house. We made attempts to get help from our GP at the time because it was so far from the norm that we were worried. My son had blood work done, stool tests, and barium swallow to try and find what was causing his problems. tests showed nothing. AFter the third episode in 2months, I resorted to taking him to a Chiropractor/ Naturopath who put him on Ultra Flora Plus and Probioplex. His issues went away. Months later(no longer taking the the Flora and probiotics) the vomiting and diarrhea cycles surfaced again. By this time, would had moved across the country and no longer had the support of the doctor or access to the certain brands of probiotics that we had been using. Another symptom surfaced shortly after… REALLY bad sulfur burps that would proceed the vomiting and explosive diarrhea. There hasn't seem to be any rhyme or reason to set off his episodes over the years. Over time they have been less frequent but  more severe.This has been very disruptive to his life at home and school. He sometimes goes 3 to 6 months and nothing happens, then suddenly he will have an episode once a week for 3 or 4 weeks in a row. He misses alot of school because of this. For what ever reason it seems to hit the hardest in the fall months. We have had many tests run…xrays, ultrasound, allergery, EEG, small bowel follow thru, blood work, stool tests, testing for celliac. Everything comes back normal. Our Doctors and specialists have resigned to the fact that there is nothing wrong with him…I say to them TELL HIM THAT!! When he has vomited in his bed, or running to  the bathroom at school hoping to make it. No he is not sick everday.. but does he need to be to make finding out what is wrong important? I wonder is there such a thing as taking to many probiotics and can you become dependent and need to take them to have a healthy digestive system?