We have had similar issues with Rocket. Our vet diagnosed it as stress colitis, since every time it happens he is boarded or there is some triggering event that we know of. We have just started Tylan and are seeing good results after only a week. Metronidazole has been the go to drug for us for a long time, but just wasn't doing anything this last time.
My usual recipe for him when this happens is very mushy over-cooked rice (6 or 7 cups of water to 2 cups of rice) mixed with Evanger's Organic Canned Chicken. It is very easy on his tummy and after a short time of a week or so I can start working him back on to kibble. I also feed 4 times a day instead of two when using this so ;I'm not just shoving food through him. His stomach seems to handle 4 times better than 2, on soft food or on kibble. I will also usually make a huge batch of scrambled eggs and mix this in a couple of the meals each day as well.
I prefer the Evanger's organic canned chicken instead of cooking chicken for him. it is very finely ground, easy to digest, and contains only organic chicken and a little bit of organic Guar Gum as a stabilizer. It does not have all of the chemicals and preservatives that the canned ID food has, and on a per can basis is about 1/3 to 1/2 less in price. .
The ZD is pretty much junk. Rocket refused to eat it the time that we tried it.
As far as kibble, we had good results with Precise Sensicare Lamb Meal and Rice which is made for sensitive stomachs. I only changed because we couldn't seem to feed him enough of it, He was always hungry. I switched to Precise Senior and he has done well on it, although we are continuing to monitor him this time as he goes back on it to see if it may be time for another food change.
So far, the Tylan seems to be doing the trick.