I think he is probably full of rice..... I never really thought about that but perhaps since you have been going bland so long there is just too much rice in his system?
Hmmm..... let me think about this one. Perhaps 1/2 rice and 1/2 kibble would be enough. I feel so bad for you, you have gone through a food switch nightmare. It should never be this difficult. I think Duke is just very sensitive in his tummy like my Jesse was.