New treats could cause issues but if started slowly, it should be okay.
He must be very sensitive.
With very dark stool, there is the concern of internal bleeding (which doesn't sound like the case here). If the food isn't agreeing, it needs to be switched but it will be difficult because everything seems to bother him. If he is that sensitive, I'd wonder if a prescription diet will have to be considered because you can't let the runny stool get out of control or he could dehydrate. That could easily happen.
Canned pumpkin may firm his stool up but it would just be covering the issue up, so I wouldn't be comfortable using it long term.