Non-solid, sickly sweet-smelling stools are characteristic of coccidiosis. Give your vet a call and ask, first of all, if you can make an appointment for the cat to be seen, but ask if he can run a fecal sample first, because if it's coccidiosis, a course of Albon will fix him up and if you have a decent relationship with your vet, he may just let you pick up the Albon and forego the physical exam to save you a few dollars. If the fecal comes up negative, then you're going to want to have him checked out to rule out other causes.
Ghilly
Let's start by giving him wet food along with the dry for more complete nutrition.
Has he been checked for the usual diseases - FIV etc, internal (and external) parasites... that sort of thing and has he been innoculated?
How old is he? Has he been neutered? What brand is the food (is "all cats" a brand name?)?