This is not possible with Pro Maps for Google. It looks like he is using a custom script to manipulate the info window:
<script type="text/javascript" src="http://static.travbuddy.com/scripts/travmarker.js?version=7.06"></script>
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.