I think you would just have to add this to your query:
AND ProductManufacturerID = ?
and then add the parameter:
$Categories->bindParam("s", "".$_SESSION['SearchMan'] ."", "1"); //manparam
And then maybe add this to the top of the page in case they hadn't set the manufacturer yet:
<?php
if (!isset($_SESSION['SearchMan'])) $_SESSION['SearchMan'] = "1";
?>