Lara firstly many will understand how you feel, I'm always in tears if one of my cats are ill. Personally I think you should grit your teeth and ring the vet now, ask them to explain why they gave you that particular antibiotic and what they think it is treating, tell them he's not eating or drinking today, they should take that seriously. All the symptoms seem to point to a tooth/abscess problem (to my untrained eye) but as they have ruled that out it can't be. Ring them, it will stop you worrying about doing nothing but worry.