Long IBS Diagnosis

Back in August I lost 10 pounds over the course of one month. I suspected it was due to the digestive issues I've been having for 4 or 5 years now. Every time I eat or drink pretty much anything other than water, I have a case of diarrhea within no more than 20 minutes. I believe I have IBS, but when I went to the doctor, she told me she thought it was a MINOR case of IBS. I don't know about y'all, but I consider losing 10 pounds in one month (went from 127 down to 117lbs) a little more than minor. I was sent to a gastroenterologist who has a "suspected diagnosis" of IBS, but it's been 6 months and he still hasn't officially diagnosed me. I know doctors like to do thorough testing so as not to get sued, but the extent of the testing is too far. I've had blood work, stool cultures, a colonosocopy, and an ultrasound, but the doctor wants me to do even more testing (a second bloodwork and stool culture just to "make sure"). I'm a college student and honestly can't afford all this testing and the only treatment my doctor has offered is a high fiber diet and an over the counter probiotic, neither of which help. What can I do to make my doctor prescribe me something for IBS and make him stop running unnecessary tests. My diagnosis has taken too long and I think my doctor is not taking me seriously.