Molting is controlled by a wide range of factors including thyroid activity, reproductive hormones, photoperiod, body condition, age and diet. After a series of molts to attain adult plumage most birds go on to molt 1-2 times annually. The pattern of molting is orderly and in the following progression (with some overlap): the inner primaries; the outer primaries, the secondaries and tail feathers; and finally the body contour feathers. It's usually bilaterally symmetrical and is paced so as to avoid loss of flight capacity at any time. A normal molt might take 3 weeks to 3 months to complete.
If his molt is delayed, first examine his diet. Which type of bird is he? What has his diet consisted of, please? 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.