Advice on my guinea pig?

I've got a male guinea pig and he is around 2-3 years old. Lately I've noticed he hasn't been leaving his igloo much and is not eating. He's got very chubby and has a slight bit of hair loss behind his ear. Usually, he used to run when you tried to pick him up, now, he just sits motionlessly; as if he doesn't have the energy. I would appreciate some answers, asap.