OP, take your little one to the vet, they can get dehydrated so fast. If left untreated, the bacteria can grow & cause more problems.
Colitis can be stress related & can be managed w/a low fat high fiber diet. That's good that he's not vomiting, as that can be something more serious like pancreatitis. Princess just recovered from a bad colitis episode couple of months ago. She was very sick over the weekend & I waited til weekday to take her bc her condition wasn't improving @ all. I will never delay taking her in again (oh how much she had to suffer

)! The vet gave her subcutaneous fluids, penicillin & VitaminB shots. Her stool improved after taking 2 doses of flagly/amoxicillin. It would be helpful to tell the vet which if any antibiotic he's had last, as they rotate it.
That place has to be reported, did you talk to the manager or owner of the place? I don't understand WHY they didn't call you...