Just echoing everyone here. Fluid from his nose is classic sign of a URI. It is good that he is drinking and eating, but being a bit lethargic and inactive is a big huge warning sign. Pigs are prey animals and don't let on that they are sick, unless they are VERY sick. He could deteriorate very quickly, seeing a vet as soon as possible is vital.
Please see this before you take him in, so you will know what meds are safe:
Guinea Lynx :: URI