If your guinea pig is exhibiting these symptoms, he needs to be seen by a vet right away!
There could be many causes for such symptoms; old age, poor diet and a range of injuries, illnesses or diseases.
How old is he? Guinea pigs usually only live 5 to 8 years, even with the finest of care. It is not unusual for an animal to become lethargic, weak and disoriented in old age. If he is over 5 and taking him to a vet is not an option, just do your best to make him comfortable. Elderly animals often have a hard time regulating their body temperature too, so make sure he's kept in a warm, non-drafty area.
Has he been eating a nutritious diet? The symptoms described could also be signs of advanced malnutrition, or even dehydration. Unfortunately many generic 'small animal food' mixes are less-than-ideal as far as meeting specific nutrition needs for many species. Products like Oxbow's Critical Care for herbivores (here, for example http://www.arcatapet.com/item.cfm?cat=17858&source=GA-PLA02017858) will boost nutrition, supply the fiber herbivores need for proper digestion, and will help improve hydration.
It is difficult to say more without knowing more details. Again, the best solution is to get him to a vet if at all possible. I hope everything works out for you both!