Depressed guinea pig?

I've had my male guinea pig for about 2 years and I'm starting to worry about him. When he was younger he loved to play and run, but for the past year he has gained weight and is scared of everything. When I try to let him run around, he sits still and squeaks or runs under me and hides. He does no physical activity! I've let him go outside, in my room, everywhere. He just sits. He doesn't like being taken out of his cage, even though I"ve never hurt him or anything. He also will not associate with other pigs if he sees them. I just want him to be happy. Any tips?