add_filter('post_type_link', 'custom_product_link', 1, 3);
function custom_product_link( $link, $post = 0 ){
    if ( $post->post_type == 'product' ){
        return home_url( 'product/' . $post->ID .'.html' );
    } else {
        return $link;
    }
}
add_action( 'init', 'product_rewrites_init' );
function product_rewrites_init(){
    add_rewrite_rule(
        'product/([0-9]+)?.html$',
        'index.php?post_type=product&p=$matches[1]',
        'top' );
    add_rewrite_rule(
        'product/([0-9]+)?.html/comment-page-([0-9]{1,})$',
        'index.php?post_type=product&p=$matches[1]&cpage=$matches[2]',
        'top'
        );
}