Diarrhea help...

I adopted a 5 month old Ragdoll kitten from a breeder. I researched and found that this breed (I have always adopted from shelters) would be right for my lifestyle now. When I picked him up I knew he had diarrhea but figured I would take him to my vet and the problem would be corrected. They did blood and fecal tests but all were negative. We put him on rice and turkey for a week plus that powdered vitamin stuff (can't remember the name), but no luck. We then tried a urinary wet food but then the cow pattie consistency turned into liquid. He was going in his litter box until the last 2 days and then started going all over the house. So far I spent $500.00, and came to the conclusion that I bring him back to the breeder for her to help him and I would come back to get him. I can't spend much more money on him nor do I want to clean-up poop every minute I have him. She just informed me (after a week) that he's not any better and if he's not better in a month he will have to be put down. I'm so upset, frantic and frustrated. I see many products for diarrhea on-line like the WellBeing BM Tone-up Gold. Does anyone know if that works? HELP!!!!