Senior guinea pig looking thin....

[NHWheekers]

I agree with getting him checked by a vet. I hope it's just old age, and he might do with a shot of Critical Care or Vitamin C. Alfalfa is sometimes added for ailing pigs needing to bulk up a bit. That's also something to consider.

[8ball]

Barney has lost some mass around his back and hips but he still has a chubby tummy. He eats, drinks, poos and bounds around like normal though.