The first thing I would do would be to stop him from taking/licking human food. The rule in my house is that I don't eat their cat biscuits and they don't get to eat human food.
I found with Babbage I had the same problem- soft to running poops and had him tested for parasites, which came back negative.
In the end it came down to food and seemingly being intolerant of something that is in a lot of food. I can't tell you what because I didn't definitively find out what it was. I tried Royal Canin, Applaws, Iams and several other brands of biscuits, and came to realise that any "treats" were making the problem worse. It didn't help that initially I was feeding him a mixture of biscuits so that he'd get used to having a bit of variety and not get hung up on eating only a certain brand.
In the end, cutting out the treats ( with the exception of Cosima duck bits ) and feeding him Orijen
http://www.orijen.ca/products/cat-food/?lang=en-uk solved the problem that must have been uncomfortable for him and was certainly unpleasant for me - I frequently needed to wash him clean
If it does come down to food then it will take time for things to clear up - this makes it hard to get a food that works for Clive as you'll maybe need to feed him something
and that thing alone for a week or two before you see improvement, indeed, if you swap straight from one food to another instantly things may initially get worse.