As far as I know, there is no way to filter the Item dropdown differently for each line item. There is not a supported API for filtering a native dropdown.
The closest feature that comes to mind that is built-in to NetSuite is the Item Filter functionality, but this filter applies to all line items and is not dynamic.
You can investigate the Item Filter functionality to determine whether it works for you by looking at your custom Transaction Form > Screen Fields > Columns tab > Item Filter. This allows you to select a Saved Search that will determine what Items show in the list for that form.