There are a number of reasons why your cockatiel might seem like he’s always angry. If he’s suffered some sort of traumatic experience in a previous home, he could be frightened that it might happen again, and as such is always in defensive mode. The only way to alleviate his anger, which is really fear presenting itself as anger, is with time and by giving him the space to learn to trust you.
The reason for his anger could also be due to a lack of socialization, and the fact that he’s just not used to being around humans and doesn’t know how to react in their company or presence. Again, the best way to help your cockatiel to feel more secure and less frightened, and to help him calm down is by giving him the space and time that he needs to learn to trust you and accept that humans won’t hurt him and that he’s now part of your family.
However, your cockatiel might be angry because he’s jealous. If he’s bonded with you, and sees you as his mate, spending time with other people while you’re in his company could be enough to make him angry, and the more time that you spend with others while he’s around, the angrier he’ll get.
If this is the case, you’ll need to try and teach your cockatiel to become less dependent on you, and the simplest way to do that is by encouraging other people, whether family or friends, to also spend time with him to help him to expand his social circle and push him to interact with others. It will take time, but with patience and understanding, your cockatiel will eventually begin to calm down.