Oh my goodness that looks awful! What I can't figure out for the life of me though is why he won't perscribe ivermectin even as a preventitive, it is very safe, I have never heard of a vet overdosing a pig or harming a pig with it! What could it hurt? I would find another vet, or seriously give self dosing another thought.
This may help clarify a few things about treating at home for you;
Guinea Lynx :: Topic - Ivermectin usage ~ links and pictures included
Or, maybe call another vet and see if they are willing to give your pig a dose of revolution as a preventitive measure.
Skin scrapes often reveal false negatives, they are not reliable.