The ID of the parent post is in $post->post_parent
. When a post has no parent, that number is 0
. So …
if ( is_singular() )
{
global $post;
if ( 0 !== (int) $post->post_parent )
{
$some_value = get_post_meta( $post->post_parent, 'some_metabox_param', true );
if ( ! empty ( $some_value ) )
echo $some_value;
}
}