your results page is not actually using the data Assist Search, it is just using the URL variable being passed in by the form.
in your serach form you have the action sett to post to the results page, and you are using the recordset to pass the ID:
<form name="form1" action="product_results.php?CategoryID=<?php echo $row_rsCategory['CategoryID']; ?>" method="post">
this ID will always be for the first record.
change the action and the method of the form to not pass the URL in the action, and use the GET MEthod:
<form name="form1" action="product_results.php" method="get">
By using the get method, the form elements will now be passed as the URL variables.
change the name of the select list from Search
Search to CategoryID:
now the CategoryID URL variable will be controlled by the select list instead of the recordset.