Check his mouth again. Thoroughly with a flashlight. He might have something stuck between his teeth that is rotting? Or a baby tooth retained that was missed by the vet? Or a cracked tooth? If his mouth is totally clear, check his anal glands. If they are bothering him, he will lick at them (sometimes when you aren't watching). What are his poops like? Normal? If so, then I doubt he's having a tummy problem. But you could try a probiotic anyway just to see if it helps. I tend to like the ones for people that are kept in the fridge in the natural foods stores. They have the highest counts of beneficial bacteria. Yogurt is just about useless and the pet ones tend to be the same. Get a really good human one and try that.