Select field, show only child options

I have this working code:

<form role="search" method="get" id="searchform" action="">
<div>

<strong>Search for:</strong>
<strong>Type</strong>
<select name='s' id='type' class='postform' >
  <option value='' selected='selected'>All</option>
  <option class="level-0" value="price">Price</option>
  <option class="level-0" value="color">Color</option>&
</select>
<br></br>

<strong>Brand</strong>
<select name='s' id='brand' class='postform' >
  <option class="level-0" value="lg">LG</option>
  <option class="level-0" value="samsung">Samsung</option>
  <option class="level-0" value="htc">HTC</option>
</select>
<br></br>

<strong>Model</strong>
<select name='s' id='model' class='postform' >
  <option value='' selected='selected'>All</option>
  <option class="level-0" value="galaxy s">Galaxy S</option>
  <option class="level-0" value="galaxy s2">Galaxy S2</option>
</select>	

<input type="submit" id="searchsubmit" value="Search"
onclick="return fn_submit_search();" />
</div>

<script>
function fn_submit_search(){
window.location='/?s='+document.getElementById('type').value+'+'+document.getElementById('brand').value+'+'+document.getElementById('model').value;
return false;

}
</script

</form>

The code is working great and I am able to search in site based on pre-definited search terms.

However, on the second field I have the Brand name. On the third field I have the Product name. In this moment if I choose Samsung, for example, I am allowed to select a model from another brand.

Which solution can I use to be able to have displayed the products from a single brand, once the second field (brand) is selected?