Have you considered WHY and how important it is to him??
When you get requests like this, I found it was easy to say, "Certainly, it can be done!!"
It will be a great challenge!!! Did you want the refill to retract? Or should it stay exposed? Any other features important? I really enjoy the challenge of "inventing" new custom pens. I can either work on a "per hour" basis or as a "project". In any event, it will take several hours to design and build, so the cost will probably exceed $200 or so---is that OK with you?"
No one EVER said yes.
I did do a couple custom pens with 2-3 prototypes sent to the customer. They ran into the hundreds of dollars and tens of hours. But the customer WAS forewarned. The ones I did were designed around the refill (once) or fountain pen nib the customer liked.
But those who ordered were "pen guys", not just casual users.