Have a look on the Gorgeous Guineas Website, they have some good photos to compare.
Can you brush him lighly over a white paper to see if it anything comes off (though burrowing things wouldn't brush off, you'd need to get your vet to do a scraping of the skin)
He might just be dry and flakey with the central heating? What kind of fur do they have? What are you putting in their cage as bedding? Maybe treat both boys to a conditioning shampoo , rinse really well and dry well. It might be all he needs. Gorgeous Guineas have conditioning shampoo and melts (to dissolve and pour over the fur) If that doesn't work, then consider fungal?