Skin problems are usually either mites, fungus, or bacterial infections. While it's possible to have just one spot of mites, they're usually in a wider area. Fungus and bacterial infections don't tend to itch as much as mites, although they can itch some.
One test for fungus that sometimes works (but sometimes doesn't) is to pull on a small tuft of hair and see if it comes out. If it does, it's probably fungus. But if it doesn't, that doesn't rule a fungus out. Also, if the fungus is ringworm, the initial lesion will be round in shape.
By now you're undoubtedly getting the picture that it's hard to tell what a skin problem is unless the pig has a fairly severe case of whatever. With a bad case of mites, they scratch like crazy, and tend to lose hair in a V-shape along their backs where they bite at the mites. With a bad fungus infection, there'll be hair loss, and it will spread. A bacterial infection will also spread, but may look like open sores.
Unless he weighs less than 350 grams, it wouldn't hurt to treat him for mites anyway. The topical treatment is the easiest to do, and you can get the medicine from (broken link removed) or (broken link removed), along with the syringes needed to measure it. Instructions are at
www.guinealynx.info/mites.html and
www.guinealynx.info/ivermectin.html.
The fungus treatment is also easy, but be aware that fungus is very contagious, and you need to use gloves when handling him and change your clothes afterward. Clean the cage thoroughly, wash everything that can be washed, and throw all the wooden stuff away.
If it's bacterial, you'll need an antibiotic cream, and you'll need to see the vet to get one that is safe for pigs. Since he can reach the area with his feet, he can get the cream on them and then lick it off his feet.