My brother is trying to come to USA on students visa for masters, he is trying to change his major from Civil engineering to computer science for masters. He has already completed his bachelors in Civil Engineering, can he do so or not?
Yes, he can. However, he will have to check the curriculum and specific admission requirements for every single Master’s program he is considering to find one that’s a good match for his goals and background. Some Master’s programs will start from scratch and are targeted at students with minimal programming experience. Many programs will require a moderately-strong CS background but not a full CS undergraduate CS major. (They might give you a list of specific courses that are prerequisites for admission, such as Data Structures, Algorithms, Discrete Math and Operating Systems.)