Bad Poo

My Kitten is about 5 months old and has the stickiest, most nasty, smelling Poo I've ever smelled. I had him checked for worms and he was fine. I have been feeding him a combination of wet and dry food - different kinds - no change. All food is high end. The most recent cat food was recommended by the vet and it is W/D prescription cat food. I read that it could be Coccidia, Guardia or Tritichomanas Foetus, on this site, and haven't test for these yet. Any thoughts?

I would get him tested for the other parasites.  I would also have the test rerun for the more common parasites, they can only be found at a certain point in the life cycle.  With a kitten this is usually caused by parasites so I would make sure there are none before looking at other possibilities.

Welcome to The Cat Site, sorry it's under these smelly conditions.
I would definitely get him tested for Coccidia and the others, I think I remember reading Coccidia can produce really smelly poop.
What is your kitten's background--if he ever lived outside for any length of time, he could have Guardia (tainted water) to.
Smelly poop could be a sign of allergy, even in high ends food. Dry food has more additives than most canned food, maybe elimiate the dry food and see if his poop smells better?

I would ask for a Fecal Panel to be ran (they send it to an outside lab) to test for a variety of parasites.  If that comes back clean your cat could be having issues digesting the foods it is eating.  Have you tried grain free?