Guinea pig is sick vet doesn't know what it is..?

does he live alone? maybe hes depressed. is there someone u can borrow a pig from that he can meet through the cage (just incase its something contagious) pigs do have a habit of refusing to eat when they are ill. once they do they downhill v fast. ur doing well to keep him going so long. give the antibiotics a few days to do the trick but if nothing changes after a week take him back to the vet or maybe a different vet thats got experience with pigs. theres not much more you can do for now but wait. ur doing well dont give up now.