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A furball wouldn't be causing what you are seeing. I'm REALLY surprised that the vet didn't take a sample to check for parasites - worms or protozoan infestations which OFTEN cause this type of thing. To help right now until you talk to the vet tomorrow, I would give him 1/2 of an immodium a/d. This should help. I would also withhold food for about 12 hours to give the gut time to rest. Don't give him Pepto Bismol as it contains aspirin which is toxic to cats. If he appears dehydrated at all (skin does not snap back quickly into place when you pull it up between the shoulder blades), I would push fluids - unflavored pedialyte is always a good choice.
My thoughts on this (without seeng him or a stool sample are):
Roundworm, Coccidia, giardia, camphylobactor or clostridium. All of these, with the exception of roundworm are treated with a course of flagyl for 14-21 days. Round worm is treated with a dewormer containing pyrental pamoate. You can purchase this at the pet supply without a prescription.
He may also have an acquired food allergy. It's not unusual for cats this age to develope a problem with corn or corn gluten. I would try gradually changing him over to a corn-free diet, such as Wellness, Innova, Felidae or California Natural to name a few. Most commercial cat foods do contain corn or corn gluten, but not all of them. You need to check labels and move him over gradually by adding a little new food to his existing diet, and each day add a little more new food and a little less old, until he is eating ALL new food in about 7-10 days.
For today, just to help with his discomfort, I would use the immodium (no more than once every 12 hours and for no more than 2 days). Fast him until tomorrow morning and then gradually start him back on food. You might even purchase some plain Gerber Baby Meat (chicken or turkey) as these are bland and should set well with him. You can keep him on that for several days if necessary and then start weaning him onto the new corn-free diet. That should help quickly.
Please let me know if I can assist further.