Please tell me that the yogurt was NOT a diet variety. Artificial sweeters are toxic to our pets

If you want to give yogurt, that's really good for them, but it should be the plain, unflavored variety. Try to stick with the regular or lowfat, but never the fat free (it's full of thickeners and stuff that's really not good for any living being

!)
If you give him a new food, make sure that you just give the tiniest smidge at first, to make sure he tolerates it well.
Is his ear red inside, or does it stink? Do you see any black dots, that look like dirt? Could be ear mites, if so. Need to call the vet first thing tomorrow if you suspect any infection there so it can get treated immediately!
I hope he feels better, poor wittle guy