I have this issue where I need a CPT to have several layout options and use templates. Originally, WP does not support templates for custom posts, however I know its possible to do it manually via Meta boxes custom data.

This is the snippet of code I have in my functions, which enables possibility to choose template and save it. It works fine – adds data to db and saves/updates the template. My issue is in the last part, where I try to redirect the theme to correct template and some-why it does not work.

 // --------Add actions
 add_action('add_meta_boxes', 'custom_template_add_custom_box');

 /* Do something with the data entered */
 add_action('save_post', 'custom_template_save_postdata');


 // --------Add functions
 /* Adds a box to the main column on the Post and Page edit screens */
 function custom_template_add_custom_box() {
     add_meta_box( 'custom-post-type-choice', 'Choose custom template', 'custom_template_inner_box', 'campaign', 'side' );
 }

 /* Prints the box content */
 function custom_template_inner_box( $post ) {

     // Use nonce for verification
     wp_nonce_field( plugin_basename(__FILE__), 'custom_template_noncename' );

     if ( 0 != count( get_page_templates() ) ) {
         $page_template = get_post_meta($post->ID, '_wp_page_template', TRUE); ?>
     <p><strong><?php _e('Template') ?></strong></p>
     <label class="screen-reader-text" for="page_template"><?php _e('Page Template') ?></label>
     <select name="page_template" id="page_template">
         <option value='default'><?php _e('Default Template'); ?></option>
         <?php page_template_dropdown($page_template); ?>
     </select>
 <?php }
 }

 /* When the post is saved, saves our custom data */
 function custom_template_save_postdata( $post_id ) {

     // verify this came from the our screen and with proper authorization,
     // because save_post can be triggered at other times

     if ( !isset( $_POST['custom_template_noncename'] ) || !wp_verify_nonce( $_POST['custom_template_noncename'], plugin_basename(__FILE__) ) )
         return $post_id;

     // verify if this is an auto save routine. 
     // If it is our form has not been submitted, so we dont want to do anything
     if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) 
         return $post_id;


     // Check permissions
     if ( 'page' == $_POST['post_type'] ) {
         if ( !current_user_can( 'edit_page', $post_id ) )
             return $post_id;
         }
     else {
         if ( !current_user_can( 'edit_post', $post_id ) )
             return $post_id;
     }

     // OK, we're authenticated: we need to find and save the data

     $page_template = $_POST['page_template'];

     if ( !empty($page_template) && 'campaign' == get_post_type($post_id) ) {
         $page_templates = get_page_templates();
         if ( 'default' != $page_template && !in_array($page_template, $page_templates) ) {
             if ( $wp_error )
                 return new WP_Error('invalid_page_template', __('The page template is invalid.'));
             else
                 return 0;
         }
         update_post_meta($post_id, '_wp_page_template',  $page_template);
     }
 }

 /* Custom template redirection hook */

 function custom_template_redirect() {
     global $post;

     if ( is_single() && 'campaign' == get_post_type($post) ) {
         $page_template = get_page_template();
         include($page_template);
         exit;
     }
 }
 add_action("template_redirect", 'custom_template_redirect');

So how can I ‘use’ the selected template for that page?