First of all, switching food is definitely a good idea since Iams isn't very good.
However, I doubt that food allergies would be the problem, especially at only 9 weeks old. Parasites are a possibility or coccidia (read up on that here:
http://www.peteducation.com/article....&articleid=727 Gotta love Google!). Anyway, I know that stress can be a factor with coccidia, especially when switching homes and what not.
I would take your little guy to the vet to get him checked out. He could have a bug, and if so, you probably would want to get it taken care of ASAP. And if he doesn't, better safe than sorry.
Hope this helps! You might also do a search through the forums for diarrhea. A lot of peoples' furbabies seem to have that problem at some point or another.