Oh, I really hope and pray you can catch him and he's safe. I know this probably sounds strange, but I have had luck a few times catching parakeets that flew out the door of neighbors by quickly throwing a lightweight baby blanket over him (or a dishtowel or whatever you have that is a size that would work) over the bird if it's on the ground, especially after it rains, as their feathers are too wet for them to fly up, especially if his tail is wet. You just quickly try to judge where the bird is under it, and calmly grab him, get him in the house. If it's a large towel you can even throw it over the bird, fold it quickly and get him in the house. It has worked for me. It also worked for us to get a parakeet we had that flew out of the house after a rain storm. I don't think cockatiels can fly when they are very wet either. Other than that, you are doing right by putting the food, water, etc. in a cage. at least he is probably eating. And him hearing another cockatiel chirping may also help him and convince him to come. But since winter is coming, it';s getting cold, if you have the opportunity, try to grab him with a towel. It will take him by surprise, you may have him in your house before he even has a chance to react. good luck, prayers.