Giardia is very hard to diagnose by a regular fecal exam. Did they just do one fecal float or did they charge you for the more expensive stain test? Are his stools light in color and greasy looking? If so, suspect giardia. I think it's been too long to just be a simple stomach upset when all he has been eating is his regular food. There have been some cases of contaminated food - did you recently open a new bag when the problem started. I would think he has some type of bacteria or parasite in his digestive tract. Talk to the vet about how long he can go on with diarrhea before he wants to put him on Panacur or Flagyl or some other antibiotic. In the meantime, give Pedialyte to restore the electrolytes lost with the diarrhea and feed the boiled chicken (easy to digest) and white cooked rice to firm up the stools.
Here's a link to giardia.
http://www.thepetcenter.com/exa/gia.html