You just need to do a join in your trigger.
Something like this should work:
UPDATE Stock
SET StockQuantity = StockQuantity - o.ItemQuantity
FROM Stock s
INNER JOIN inserted i ON s.ProductID = i.ProductID
INNER JOIN Order o ON s.ProductID = o.ProductID
But my guess is you have lots of orders so you may have to do something like
UPDATE Stock
SET StockQuantity = StockQuantity - SUM(o.ItemQuantity)
FROM Stock s
INNER JOIN inserted i ON s.ProductID = i.ProductID
INNER JOIN Order o ON s.ProductID = o.ProductID
Inner Join to the inserted table in your trigger so that only those records that are being changed will be updated. Or, remove inserted to do the entire table.