It will take a while for you to see the full effects of the switch over from IAMS to the Fromm. So I wouldn't give up hope on the Fromm just yet. There will be others on here who are going to give you some really good advice. @desertskybulldogs will be able to give you some good info. It may even be worth trying the bland diet for a day or two to see if that will clear up his upset stomach before moving him back to the Fromm.
How is his health generally? Is he still playing normally and looking healthy?
Has Bruno had all his stool sample tests done? Could it be giardia or some other intestinal parasite that is giving him the diahorrea??
Just some other thoughts that sprung to mind ......