The problem you are fighting is a (nothing personal) genetically inferior animal. I'm sure you know that Skinny Pigs were bred for their lowered immune systems. You are fighting an infection in an animal that was bred to be infected.
Are you certain there aren't ear mites present? The anitbiotics won't kill the ear mites and the earmites will cause the infection to keep coming back because to the irritation they cause.
Make certain the piggie is kept warm and away from all drafts. Maybe if you add some immune system building veggies to his diet that wll help. I believe veggies like Brussel Sprouts, Cabbage, Broccoli, anything with high Vit C content, squash, sweet potato, parsley...
Of course some of the veggies listed must only be fed to a GP in moderation so be sure to research on http://Guinealynx.info
all of these veggies and others before feeding.
The links below are good reading and may help you find more foods to offer. The last one also talks about accupressure points to use. It's for people but those spots exist in animals too. Read the description of where they are on humans and try to find them on your GP...massage those areas several times a day to try to get him to boost his immune system.
I have used accupressure on my animals in the past and it always had good results. You may even want to look into finding an anmal accupuncturist. Believe it or not they can sometimes offer relief when western medicine fails.