Diarrhea cat!

I have had my 2nd cat, Sam, for almost 5 months. Almost every bowel movement of his is diarrhea. I tried different diet options (all dry, combination dry/wet) and it doesn't seem to make a difference. His stool sample at the vet had no issues either. What else can I try? I can't believe it's comfortable for him and, honestly, I'm getting sick of the stench

The only other thing strange about Sam is that he seems to be lacking any feeling in his tail - he never moves it and doesn't care a bit when we accidentally step on it/poke it. No reaction at all. This isn't a huge concern for me, but could it be related? (His tail is otherwise healthy. Great coat etc.)

It is very concerning when you say he seems to have no feeling in his tail. How about the lower part of his body? Did your vet address this issue?
Where did you get the cat and how long have you had him? The reason I ask is when we adopted our cat, December, he had the same issue. He had been caught in a trap, taken to a shelter, and then to a pet store. His stool at the vet showed no parasites, etc.

One thing that might work is a probiotic which can be sprinkled over his food.

I solved the problem by putting him on a limited ingredient diet. He eats dry Green Pea and Duck cat food from the pet store or bought online. It is a grain free food.

I hope you get the problem solved. Did your vet suggest anything you could try?

Diarrhea can be caused by many things--diet is usually number 1 culprit if the cat shows no other obvious signs of illness. Make sure kitty stays hydrated though--diarrhea can cause dehydration quickly and that can cause diarrhea. There is a drug you can get to stop diarrhea--metronitozole, but you need to speak to your vet about that. one pill usually works.

Good luck and get baking soda--neutralizes smells. And light a match then put it out--sulphur takes away smells too.

If kitty doesn't move his tail, it's possible it may be damaged---best to ask your vet.