Hi there,
I will try to answer your question. One very common reason that hair loss, of alopecia, that occurs in guinea pigs is a mite known as trixacarus. In early stages, pigs may be asymptomatic (not itching, scaling, etc). Mites are "normal residents" to many species including pigs- sometimes with stress, age, concurrent disease, etc, these mites can begin to replicate and cause clinical signs. They are typically treated with topical Revolution. Pigs can also get fleas, nits, and lice. Another concern would be a fungal infection such as ring worm. In the guinea pig, you do not always see the classic "ring" structure that occurs in people and this is typically not an "itchy" disease in the pig. This can be treated with an oral liquid medication. Lastly, in female pigs a common reason for alopecia is hormonally related but if you are certain you have a male obviously this would not be a consideration. I suggest bringing your pig to your veterinarian for skin testing including a fungal culture and possible skin scrape. Also, some parasites may be visible to the naked, trained eye and your veterinarian may be able to diagnose based on physical examination alone. I hope this information helps.
-Dr. Jessica