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It sounds like your guinea pig might have a respiratory infection and possibly pneumonia. He is probably not eating because he doesn't feel good. You really need to take him to a vet. They will probably put him on antibiotics. This doesn't mean he is dying, but he could die from this. Respiratory infections can turn in to pneumonia very quickly and that is a lot harder to treat. Guine pigs usually live for 4-7 years and his age can be a factor in why he has gotten so sick. Please take him to your vet. He sounds very sick, and the quicker you act, the better his chances are to survive this.
I hope this helps!!