Simplify the following text for a Grade 13 native English-speaking person.
Original: He performed at the world 's major opera houses , including the English National Opera , Royal Opera House , Covent Garden , La Monnaie in Brussels , La Scala , Milan , the Metropolitan Opera , New York , the Vienna State Opera , and the Paris Opera .
Simplified: He sang in opera houses all over the world , including including the English National Opera , Royal Opera House , La Monnaie in Brussels , La Scala , Milan , the Metropolitan Opera , New York , the Vienna State Opera , and the Paris Opera .