PLEASE READ? GUINEA PIG?

Some times growing up is a bit hard to do. You answered your one question with the statement "I checked on him a few days ago." You can't leave guinea pigs alone for a few days, look what happened when you did. Your guinea pig is dieing of an upper respiratory infection that is to late to treat, however, if you had been there every day, several times a day like you are supposed to be you would have seen him becoming ill and gotten him the medical help he needed.