Yes, it does rather sound like he can’t digest his food properly.
The chicken and rice diet is good and bland but if he has been on it for the last 2 weeks and still has runny stool, then it has to be something in his digestive system.
Maybe some probiotics would help? Maybe feeding him one meal a day instead of two might make it easier for his system to process the food?
Has he had bloodwork done? If so, did it show anything?
Could he have gotten into something poisonous 6 weeks ago when it started?
I hope you can get him healthy again. It’s frustrating to have him doing so poorly.
Common sense isn't so common these days.