There's a good chance that this parakeet is simply exhausted from being far more active after escaping his previous home. He needs to be placed in a dimly lit unlikely to be disturbed area until he rests sufficiently. He's likely to be malnourished at this time and so will need an exemplary diet. His droppings reflect whatever he's been eating outside. The brown color isn't pathognomonic (specifically indicative) of any one particular disorder. Seeds should compose less than 20% of his diet. A diet of mainly seed and nuts has excessive fat, carbohydrates, and phosphorus; marginal protein; adequate vitamin E, and are deficient in amino acids, calcium, available phosphorus, sodium, manganese, zinc, iron, vitamins A, D3 (necessary for efficient absorption of calcium), K, and B12, riboflavin, pantothenic acid, choline, and available niacin. Ideally, a balanced pelleted diet such as can be found here: www.harrisonsbirdfoods.com or here: www.lafeber.com/pet-birds should be fed as well as hard boiled egg yolk, pancakes and cornbread, the tops of fresh greens, dairy products such as yogurt and cheese, fresh fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat.
I applaud your rescuing him! Please respond with further questions or concerns if you wish.