Then he sang at Milan 's La Scala ( 1951 ) , the Glyndebourne Festival ( from 1954 on ) and London 's Royal Opera House , Covent Garden ( 1954 , 1956 and 1958 ) . He subsequently sang at Milan 's La Scala ( 1951 ) , the Glyndebourne Festival ( from 1954 on ) and London 's Royal Opera House , Covent Garden ( 1954 , 1956 and 1958 ) .