Hi Pete,
I've collected Rotring for several years, and they don't make many pencils. The only one I know of that might be taken apart like you wish would be the old style Rotring 600. Maybe the new style, but I haven't seen one in person.
The old style 600 pencils can be found on sale/close-out for less than $50, but you'll have to look around. Several of the bigger pen dealers have dropped Rotring, and many never sold their pencils, just their pens.
Daly's http://www.dalyspenshop.com/store2005/product.asp has the old style on sale for $44.95.
Montgomery's http://www.montgomerypens.com/search_results.asp?collection=Rotring%20600 has them for $49.95. They also have the new style for $29.95 and up, depending on platings.
It looks like all Rotring products on Amazon are listed as unavailable.
That's a lot for a "look and see if it will work" adventure. And I don't mean to imply that any of them will work. I don't know. But those are the only ones I'm familiar with that might work. I don't have any Rotring pencils in my collection, so I can't go look at one for you. The body is a hex shape, so you'll probably want to put it on a metal lathe and turn it round before adding a wood sleeve.
One pencil that you might be able to switch him to is the Koh-I-Noor Rapidomatic Fine Line. It's better quality than the Pentel pencils, and might work. Cost is just under $10, look on Amazon. Again, you'll probably have to machine the body round to add a wood cylinder.
Sorry about that,
txbob