Echo custom post field in shortcode

Hello everyone,

I have made a custom post type, which is called condoleance.
Also, I made a custom field for that custom post field, named date_of_birth.

Now, I want to display all the condoleance posts in a table.
That is going really well, but WordPress won’t echo the date_of_birth of every post.

Does anyone know how to do that?
This is my code:

$query = new WP_Query( $args );
        if( $query->have_posts() ){
            $string .= '<table class="condoleance">
							<tr>
								<th>
									<strong>Naam</strong>
								</th>
								<th>
									<strong>Overlijdensdatum</strong>
								</th>
							</tr>
						';
             while ( $query->have_posts() ) : $query->the_post();
                $string .= '<tr>
								<td class="condo-info">
									<a href=" ' . get_the_permalink() . ' ">' . $image . '
									' . get_the_title() . '</a>
								</td>
								<td class="condo-overlijden">
									' . get_post_meta($wp_query->post->ID, 'date_of_birth', true) . '
								</td>
							</tr>	';
			endwhile;
            $string .= '</table>';
		}
        wp_reset_query();
        return $string;

Regards,
Joren