My cat's stool smells like death...PLEASE HELP

I adopted a 7 month (now 10 months) old male short hair back in December. I suppose I should mention that he's MY first can't. So I'm still on a learning curve - though I've lived with cats all my life. As long as I can remember his stool has smelled awful and I'm at a loss of what to do. In other words, I'm still convinced that they can't naturally smell this bad and something is wrong. It's so bad that it can clear out my entire apartment. As I said, I've had other cats before and they've never smelled this awful.
When I first adopted him 3 months ago, I took him to the vet. So he's cured of fleas and ear mites...if those could even remotely be a cause. I've changed his food recently to Nature's Recipe Grain Free Indoor to give him a higher quality food and see if that helped. His stool has never been normal in terms of physical appearance either. Never really loose, but still too soft. So I did a some research and found that he could have worms. I took him to the vet to get a fecal sample and his monthly Revolution. The fecal came back negative (of course), but the next day found a tape worm in his stool. Just one. I went to Petco and bought Trade Winds tape worm tabs because they have excellent reviews everywhere. I gave him one as the directions stated and the next day only found ONE more in his stool. I could be wrong, but I always that where there's one worm there's a lot more. So I'm confused as to way I've only found two. Unless I just don't understand how the tablets get rid of them.
As you can see, I've been proactive in trying to solve this terrible mystery. PLEASE help. It's hard to have company over or to even visit home with him because they smell that bad!!

If he had fleas it's pretty much a given that he will have tapeworms.  You are actually probably finding sections of tape worm.  The float test that the vet does is kind of hit or miss, it will only find worms at a certain point in their life cycle.  Since this is a long standing issue and you have tried diet change I would ask the vet to have a fecal PCR test done.  It has to be sent out and takes about a week to get results back.  It is also more expensive than the float test.  It is more accurate and will check for parasites that the float test doesn't find.