Our 4 legged dog, Ruby, has a very sensitive tummy.  We got her allergy tested and found out she is allergic to a lot of different foods (all grains, fish, rabbit, venison.... the list goes on).  Once I eliminated them from her diet her stools firmed up nicely.  There have been occasional relapses, usually when she scavenges something she shouldn't like a potato chip or biscuits (she is allergic to both wheat and potato).  Pumpkin and white rice don't work for her either because she is allergic to them as well.  If she has a bad relapse I give her a Flagyl tablet and that usually has her back to normal.

I also thought that basmati rice had the highest GI index of the white rices? So maybe try that as a compromise?  Or as Ralph suggests, try cooking the brown rice for longer??  Although maybe the extra roughage is too much?  So maybe oatmeal might be better instead of the brown rice. It might be gentler on his GI system.

Ruby is also taking some probiotics with every meal as well.  And she can even eat raw bones again (as long as I don't leave too much fat on them)!

Good luck!  I hope you can avoid a poo-pocolypse!

Karen