After new row INSERT statement, use newly created PK as FK in a second INSERT statement within the same Stored Procedure Using SQLExpress --

I have a table Orders that contains a PK of order_id. I have a second table Items that contains model and quantity along with a FK of order_id.

I am using a gridview to collect model/quantity data for an order and then I want to take that gridview data row by row and insert a new row into Orders, then insert it into the Items table, using the same order_id for each row of the Gridview. Problem is that the order_id doesn't exist until the first Insert is done. 

After researching I had hoped I'd found a solution but it is not working as expected. I don't think the title of my question makes sense but I have no idea how to word it. Thanks in advance!

UPDATE: I have tried the edits suggested in the comments and referenced the link provided but still have not gotten my answer. 

Below is my code:

Gridview Code:

&lt;asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" HeaderStyle-HorizontalAlign="Center" HeaderText="Assigned" HorizontalAlign="Center" Width="346px"&gt;
&lt;Columns&gt;
    &lt;asp:BoundField DataField="Manuf" HeaderText="Manuf" /&gt;
    &lt;asp:BoundField DataField="Model" HeaderText="Model" /&gt;
    &lt;asp:BoundField DataField="Qty" HeaderText="Qty" /&gt;
    &lt;asp:BoundField DataField="Notes" HeaderText="Notes" /&gt;
&lt;/Columns&gt;
&lt;/asp:GridView&gt;


Code behind to call Stored Procedure:

 protected void CreateOrder_Click(object sender, EventArgs e)
        {
            string connectionString = ConfigurationManager.ConnectionStrings["HMTDBCS"].ConnectionString;
            using (SqlConnection con = new SqlConnection(connectionString))
            {
                foreach (GridViewRow g1 in GridView1.Rows)
                {
                    SqlCommand cmd = new SqlCommand();
                    cmd.Connection = con;
                    cmd.CommandText = "sp_ProdDataInsert";
                    cmd.CommandType = CommandType.StoredProcedure;
                    cmd.Parameters.AddWithValue("@customer_id", TextBox1.Text);
                    cmd.Parameters.AddWithValue("@manuf_name", g1.Cells[0].Text);
                    cmd.Parameters.AddWithValue("@model_name", g1.Cells[1].Text);
                    cmd.Parameters.AddWithValue("@qty", g1.Cells[2].Text);
                    cmd.Parameters.AddWithValue("@notes", g1.Cells[3].Text);
                    cmd.Parameters.AddWithValue("@track_num", TextBox2.Text);
                    cmd.Parameters.AddWithValue("@pckg_num", TextBox3.Text);

                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();

                    MessageBox.Show("New Order Created!");
                    Response.Redirect("NewOrder.aspx");
                }
            }
        }


Stored Procedure Code:

ALTER PROCEDURE [dbo].[sp_ProdDataInsert]

@customer_id INT,
@manuf_name NVARCHAR (50),
@model_name NVARCHAR (50),
@qty NUMERIC,
@notes NVARCHAR (255),
@track_num NVARCHAR (50),
@pckg_num NUMERIC,
@order_id int OUTPUT AS



BEGIN
INSERT INTO Orders
    (customer_id, track_num, pckg_num)
VALUES
    (@customer_id, @track_num, @pckg_num)

SELECT @order_id = coalesce(MAX(order_id),0)+1 FROM Orders (UPDLOCK)
INSERT INTO Items
    (manuf, model, qty, notes, order_id)
VALUES
    (@manuf_name, @model_name, @qty, @notes, @order_id)


END
I think that if you just change this line it should work for you.

SET @order_id = SCOPE_IDENTITY();


Your method of grabbing the max id and adding 1 is not safe for concurrent access, scope_identity() is the correct way of getting the ID of the just inserted row.