Hello, I'm a small animal veterinarian and can try to help answer your question about your guinea pig.
Has there been any new pets added to the household? Any visiting children or new people living in the house?
Does his cage sit near a window where something may have frightened him?
I see he's in an enclosure with his brother. Sometimes when guinea pigs become sexually mature they will try to mate - even males on males. This may be disturbing to him if it's happening when you aren't looking. Do you ever hear them making a lot of noise, or fighting?
Please feel free to respond and I will try to further assist you