My kitten has chronic diarrhea, vet treatments aren't working. What next?

I am sorry to hear that Oliver is having prolonged diarrhea. It is good you are not ignoring it as it can be a sign of underlying issues. The steps taken so far- deworming, probiotics and diet change are all great first steps for common causes of diarrhea in kittens. As it sounds like he has had no change, I would proceed to look for less common problems. You can ask your vet about sending out a more involved parasite screen- typically sending one to the lab. I would specifically ask about tritrichomonas and giardia screening as well. In the meantime your vet may want to try a different dewormer and/or diet change (like a hydrolyzed food) as well. If all this is negative and the diarrhea persists, your vet may want to do some imaging such as an ultrasound. I hope this helps and he is back to normal soon!