First of all, set the 'fitColumns' property to true for the datagrid, and then set the 'fixed:true' for all the columns except the final column. The code looks like this:
<table id="dg" class="easyui-datagrid" title="Basic DataGrid" style="width:700px;height:250px"
data-options="
fitColumns:true,
singleSelect:true,url:'datagrid_data1.json',method:'get'">
<thead>
<tr>
<th data-options="field:'itemid',width:80,fixed:true">Item ID</th>
<th data-options="field:'productid',width:60,fixed:true">Product</th>
<th data-options="field:'listprice',width:80,align:'right',fixed:true">List Price</th>
<th data-options="field:'unitcost',width:80,align:'right',fixed:true">Unit Cost</th>
<th data-options="field:'attr1',width:150">Attribute</th>
</tr>
</thead>
</table>