Don't even let him lick the plates, as he may have food allergies that are causing the loose bowels, etc.

Buy a good lamb-and rice (usully safe for dogs with allergies) food, and make sure no one is giving him treats during the day, either. You've got to break this bad habit. He will start to eat, when his own food is ALL there is available to him. Eventually, when he's back eating normally, you could reintroduce treats and tidbits now and then, but be very careful what's in them, and watch for abnormal poop - that's a sign!