How to validate a username from database using bootstrap validator in asp.net

I want to validate a username from database using bootstrap validator in asp.net. This is some code i tried but not working... When i put remote, the username field is always invalid. Is this correct way to do so?

username: {
                    validators: {
                        remote: {
                         url: '/Registration.aspx/CheckUsername',
                         type:'POST',
                         data: '{user: "' + $("#<%=username.ClientID%>")[0].value + '" }',
                            message: 'The username is not available.',

                        },
                        notEmpty: {
                            message: 'Please fill in your username.'
                        }
                    }
                },

[System.Web.Services.WebMethod]
    public static string CheckUsername(string user)
    {
        string retval = "";
        SqlConnection bbmsConn;
        string connStr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        bbmsConn = new SqlConnection(connStr);
        bbmsConn.Open();


        SqlCommand cmd = new SqlCommand("select username from Accounts where username=@username", bbmsConn);
        cmd.Parameters.AddWithValue("@username", user);
        SqlDataReader dr = cmd.ExecuteReader();
        if (dr.HasRows)
        {
            retval = "true";
        }
        else
        {
            retval = "false";
        }

        return retval;
    }