There are so many possibilities here that a trip to the vet is the only way that you will get a diagnosis and proper treatment. A few things that come to mind in which you might see the lethargy, weakness and stumbling are heavy metal poisoning, electrolyte imbalance, dehydration, nutritional issues, kidney and liver issues, & pituitary tumor. Does he eat pellets only, what do the droppings look like? Are you able to keep him in a separate cage until the vet see him? Are you sure he is eating and not just picking?