I need veterinary advice on what to do about my dog and his diarrhea...

Ok…so I own a nine month old Peka-hund (half Pekingese, half Dacshund). He had Giardia and was treated for it, however he started getting diarrhea again…so my vet prescribed him flagyl for a week and while he was on it, he was fine and then the minute he was finished with the antibiotic, the loose stool returned. So I took him back to the vet and they put him on a higher dosage of Flagyl and prescribed a probiotic to give with it for ten days this time. Now he has just finished his second round of antibiotics, and the diarrhea has returned!!! I really can’t afford to keep doing all of these tests and having all of these vet visits, but I love him so much…I don’t know what to do! I feel like these vets I have seen are just trying to take all of my money, and my poor pup is still ill. What should I do? What else could possibly be wrong with him? Are there low cost vets in LA who really just want to help animals, or vets in LA that take payment plans? I’m completely beside myself.