Puppy has loose stool

Species: Dog

Age: 4 months

Sex/Neuter status: M/Neutered

Breed: rott/weimer

Body weight: 23+

History: adopted from kennel

Clinical signs: He has gone from blue buffalo puppy kibble to a less expensive blue buffalo puppy kibble sold at a closer location to home. I noticed his stool has changed around this time to a "soft serve ice cream" texture. This has been going on for multiple weeks. He's hyper and energetic. His gums are not pale and drink plenty of water. However, taking him out yesterday I noticed that It seemed as if was water was coming out. So the problem doesn't appear to be getting any better.

Duration: 2-3 weeks

Your general location: tx

Links to test results, X-rays, vet reports etc: He was at the vet right before the food change. Vet said hes a healthy pup. He was taken in for his 2nd round of boosters.

edit: I adjusted his diet once more. Today I went to the grocery store and purchased chicken breast and rice. Boiled it and added a little cottage cheese. I'll keep him on this and hope it's just the brand kibble that was doing it. If nothing else changes in a few days I'll schedule an appointment.