Gridview sorting with custom template fields

I can't seem to figure out how to sort my gridview with both databound AND custom fields.

The custom field look like this:

  <asp:Label ID="lblItems" runat="server" Text='<%# GetItems((int)DataBinder.Eval(Container.DataItem, "ObjectCategoryID"))%>' />

It calls for a function which shows how many item the given category has.

The sorting for the databounded fields work perfec but not the customfields. Im also looking for a generic method which works for all my gridviews.

Can someone help me in the right direction please? Below is my full customgrid code.

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;

using System.Xml.Linq;
using System.Collections;

namespace CustomControls
{
    public class CustomGrid : GridView
    {
        public CustomGrid()
        {
              PageIndexChanging += CustomGrid_PageIndexChanging;
        }        

    private string ConvertSortDirectionToSql(SortDirection sortDirection)
    {
        string newSortDirection = String.Empty;

        switch (sortDirection)
        {
            case SortDirection.Ascending:
                newSortDirection = "ASC";
                break;

            case SortDirection.Descending:
                newSortDirection = "DESC";
                break;
        }

        return newSortDirection;
    }  

    protected void CustomGrid_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        this.PageIndex = e.NewPageIndex;
        this.DataBind();
    }

    protected override void OnSorting(GridViewSortEventArgs e)
    {
        DataSet ds = (DataSet)System.Web.HttpContext.Current.Session["Source"];
        DataTable dataTable = ds.Tables[0];

        if (dataTable != null)
        {