How about tests for Chron's disease, or ulcerative colitis? Also allergy testing may reveal a food allergy that is manifesting itself by diaharrea. They can test for tons of stuff in one visit to the Dr. Are you seeing a pediatric gastrointerologist? If not, this would be the best choice.
Doc tested for celiacs (in bloods) but came back negative.He is not cosidered to be small, according to the chart (Australian blue book) he is in the 70% for weight which is above average.He's poo does smell and it doesn't bother him when he does one. He never cames and tells us unless it is very runny then he feels it. I don't know if this is also important from getting help from this forum, he had kidney failure as a baby. The tubes that come from the kidneys to the bladder, one was fully blocked the other a little but he had operations to correct this. So he has large kidneys. Also he is not regular in poos, somedays once, others none and more then once and never about the same time. So as you can imagine he is not toilet trained in this area.
Celiac disease comes to mind. It is an autoimmune disorder where ingesting gluten (a substance found in wheat and most other grains) causes the immune system to destroy the bowel lining. Is your son very thin? Does he have yellow, very smelly stools? Does he ever get a pimply rash? Try these links.
http://www.celiac.com/index.html
http://www.celiac.org/cd-main.php
It is diagnosed be physical exam and a blood test. The blood test is effective as long as he usually has gluten in his diet. Unless you've tried to avoid it, it's in practically everything kids eat. If it doesn't seem like celiac disease, look into getting the camera pill test. They have him swallow a camera pill, and it takes pictures of his whole digestive system except the large bowel. That part would have to be looked at by colonoscopy. Good luck.