Maybe he just doesn't want to date someone he has no interest in and go through the motions, or like you said maybe he is gay, but would be sad if so and felt he cannot be himself because of judgment. Count yourself lucky to have a sibling who cares enough about you to help you and try to be understanding and supportive back.
“Accept — then act. Whatever the present moment contains, accept it as if you had chosen it. Always work with it, not against it. Make it your friend and ally, not your enemy. This will miraculously transform your whole life.”
― Eckhart Tolle