Tell him sure: charge him an amount suitable for a normal nonmagical article of clothing (a few silver at most) And then say that the seamstress / tanner will need a day or two to make one for him. oh, and he'll be needing a sample scroll to get a feel for the size...
There.
Frankly, my players always come to me with all manner of I want this special clothing item that lets me do this"... and, keeping with the flavor of D&D, A well prepared adventurer is a surviving adventurer, it makes them feel satisfied if they create a new piece of equipment.
Among other things I've had:
-A wizard who tattooed select spells on private areas of her body, so she would never be completely deprived of her spell book.
-A thief with two ten-foot poles and a special screw-together attachment so they could be used to probe 20' ahead.
-several potion belts (shortly after diablo came out though, so it's not necessarily original)
And probably more "i've got a special idea" than you can shake a stick at.