function my_woo_js() {

 if( is_single('product') ) {
  echo '<script type="text/javascript">';
   echo 'jQuery(document).ready(function($) {';
    echo '$("[name=xxxx]").on("change", function() {';
     echo '$(".single .quantity, .single .single_add_to_cart_button").css("display", "block" );';
    echo '});';
   echo '});';
  echo '</script>';
 }

}
add_action( 'wp_footer', 'my_woo_js' );