My cockatiel has loose droppings that dry on his rear end and

Hello again, Joan.
Sorry for the delay, but this is a condition that is probably going to need some time to correct. Avian experts have determined that 90-95% of pet bird health issues are dietary in origin. Not only is what they eat important, but the proper proportions are every bit as important. To give some idea of how important, consider the following facts: Normally on an all-seed diet, budgies live an average of 5-7 years; with seeds and some other foods about 10-12 years; with pellets alone about 15-20 years, and with properly balanced proportions of pellets, seeds and other fresh foods, particularly dark leafy vegetables, about 25 years and even longer. The statistics for cockatiels would be even more remarkable.
Compare what you're feeding Oliver to what professional aviary keepers feed their cockatiels: 70% high quality cockatiel pellets, such as those made by Zupreem, Harrison's, Lafeber, or other reputable manufacturers like Kaytee (these may be purchased from better pet stores or online), Pelleted food is the basic building block of a balanced diet; 20% dark leafy greens like kale, spinach, Swiss chard, kale, cilantro, collard, turnip or beet greens, cooked sweet potato, squash, or pumpkin, carrots with the tops attached, dried chili peppers, yellow, green and red fresh peppers, and only 10% or less, seeds and nut foods. Sunflower seeds are very high in fat, not balanced in regard to calcium and phosphorus, and should be avoided as birds, like small children, tend to pick out and eat what they like, rather than what is good for them. Although it is green, lettuce is practically all water, of no food value whatsoever in the diet, and shouldn't be fed to birds. Fruit is okay as an occasional treat, but unless fed in a dried state, fruit contains 90% water, and quickly fills the bird up without providing many nutrients by weight. Grit is no longer considered necessary to the healthy upkeep of pet birds, and to make matters worse, grit impactions can actually be fatal.
Besides intestinal problems, fresh nutrients in balance with one another help prevent skin, respiratory, nerve, and organ problems including diabetes, as well as behavioral problems like over-zealous preening, self-mutilation, excessive or changed vocalization, excessive egg laying in females, aggression, and actually eliminate the need for vitamin supplements or tonics in most cases.
I fully understand that it may not be easy to get older birds (as opposed to starting out with a baby at weaning time) like Oliver, to switch to a better diet, or to eat the proper amounts of the different foods, but you can read an excellent article about how to teach him to eat more properly online here: http://www.cockatielcottage.net/finicky.html Commercial diets like Hartz, and other pet store or supermarket brands of seed-based foods, must be baked to kill insect eggs and larvae before shipping. This process also kills the germ in the seeds, destroying many of the heat-sensitive components in the food. In addition, these foods are rarely shelf-dated, and may be months or years old when actually fed. The result is often that the bird fills itself up on foods that contain mostly empty calories and the digested material from seeds would be very similar to the ingredients for paste, and the bacterial population that develops in the bird's GI tract tends to cause the sticky droppings. Improving his diet will most likely eliminate the need for any medications you might have used in the past, and give Oliver the very best chance at being with you for a long time to come. A product called "Johnson's Avol Mixture" is often used by bird owners to temporarily treat loose droppings. The active ingredient is similar to that found in Pepto Bismol (bismuth), so it is safe and quite effective, but it should not be used to try to treat this kind of porbem long-term, the diet and the intestinal bacterial population are the probable cause and need correction. If the loose droppings persist or if other symptoms should begin to appear in the meantime, Oliver should be examined and treated by an avian specialist. Tumors and other intestinal problems will not be helped by the above approach. You can find the avian veterinarians nearest your home by going online and googling "avian veterinary specialists+the name of the nearest large city". If you should have further questions, or if I can assist you in any way, please let me know.
Kind regards, *****