If he isn't eating, mix baby food (vegetable, with no potatoes or banana) or just mash up some bananas with water. Add some vitamin C powder, about 1/2 a teaspoon per banana or jar of babyfood. Feed it to your guinea pig through a syringe (sp?) with no needle. If he won't take it, force feed him.

Any other symptoms? Has he been in a stressful situation? Can he move his back legs?