Using custom fields to display a script in header

So basically I have a js script that I want to display only when a post has a custom field applied to it.

So lets say the script is this.js

and the custom field I used for the post i used is Trav

I put this php code into my header.php

<?php if ( get_post_meta($post->ID, 'Trav', true))
{ ?>
<script src="this.js"></script>
<?php } ?>

But then what happens is it applies to all my pages and it display all the time whether I’m in the homepage not just when I am in the post with the custom field.