Riley came to us with runny poop,s (had to put kitchen roll on them then pick em up) then had to water down the area, was that bad, we gave him chicken and rice for about 4 days, then we changed to James wellbeloved and wow, its amazing he has totally firmed up. I imagined at one point he would never firm up. Now at times its quite the opposite.
He loves the lamb, duck and fish flavour, eats it with enthusiasm each meal and has firm poops for about 2 1/2 months now (he tries to eat everything in the garden/on walks too )
Chicken is light on the stomach, I cant understand how chicken affects dogs much , tbh
You could try a few days on chicken and rice, Im sure that will firm him up and when introducing food again thats a good time to change , (our vet said this)