No menu highlight at single post pages

Link to my site: http://exogenesis.nl/
Structure: each page has its own template in which I use query_posts to show all posts from one category.

On my website, the active navigation menu item is highlighted (using the class ‘current_page_item’). But when you click on ‘Read more’ at a blogpost (and go to the single page to read the post), the menu item isn’t highlighted anymore. How can I keep the current menu item highlighted when visiting a child page? There doesn’t seem to be any ‘current’ class.

FYI

This is my navigation menu code

<?php wp_page_menu('link_after=<span class="menu_seperator"></span>'); ?>

This is (one of) my template(s) code

<?php
/*
Template Name: Politiek
*/
?>

<?php define('WP_USE_THEMES', false); get_header(); ?>

<div id="blog_left">
	<?php $paged = (get_query_var('paged')) ? get_query_var('paged') : 1; query_posts("category_name=politiek&showposts=6&paged=$paged"); ?>
	<?php while ( have_posts() ) : the_post() ?>
		<div class="content_600">
			<h1><?php the_title(); ?></h1>
			<p class="bloginfo">Posted in <?php the_category(', '); ?> on <?php the_date(); ?></p>
			<?php the_excerpt(); ?>
			<a href="<?php the_permalink(); ?>"><div class="readmore"></div></a>
		</div>
	<?php endwhile; ?>

	<div class="prevnext"> <?php posts_nav_link( $sep, $prelabel, $nextlabel ); ?> </div>
</div>

<div id="blog_right">
	<div class="sidebar">
		<h1>Categorieën</h1>
		<?php wp_list_categories('exclude=1,15,14,17,16,18,19,13&show_count=1&title_li='); ?>
	</div>

	<div class="sidebar">
		<h1>Archief</h1>
		<?php wp_get_archives('exclude=1,15,14,17,16,18,19,13&type=monthly'); ?>
	</div>
</div>

<?php get_footer(); ?>

This is my single(.php) page code

<?php get_header(); ?>

<div id="blog_left">
	<?php while ( have_posts() ) : the_post() ?>
		<div class="content_600" style="padding-bottom:5px;">
			<h1><?php the_title(); ?></h1>
			<p class="bloginfo">Posted in <?php the_category(', '); ?> on <?php the_date(); ?></p>
			<?php the_content(); ?>
		</div>
	<?php endwhile; ?>

	<div class="prevnext"> <?php posts_nav_link( $sep, $prelabel, $nextlabel ); ?> </div>
	<?php comments_template(); ?>
</div>

<?php get_footer(); ?>