I had a guinea pig that was starving and seriously underweight - not from pining but because he had a previously undiagnosed middle ear infection, which was probably making eating painful. Either way, whether starving because of a health condition or through pining, it is vitally important that he has food going in constantly. I think it would be good for him to see a vet, and they can also discuss options with you, and can provide you with liquid feed and a food syringe, and show you how to hand feed. In the mean time, you can soak some of his normal food pellets in warm water, and then either give it a whizz in the food processor or mash with a fork and feed to him with a syringe (you can wrap your GP in a teatowel, with just it's nose and mouth sticking out to make it easier). If he doesn't have food going through him constantly, his digestive system could shut down, as piggies digestive systems are quite different to many animals - they need constant food. I think you can probably save him and have a good outlook if you can hand feed him regularly, and hopefully also if you can talk to a vet. Does he have any real favourite foods that you can tempt him with? Maybe some grass or herbs? My old pigs couldn't resist a bit of mint or parsley, even when they were feeling under the weather. It would definitely be good if you can weigh your guinea pig regularly using kitchen scales, and record his weights so as to keep an eye on any change. Wishing you the best of luck x