Hi there!
I am designing a redcap that well, has not been expelled from the house, but near it.
The basic idea is that he has been cursed to have all parchment at Touch range of him rot. That includes books he might read, messages he would transport... Transforming the parchment into something else does not work, since the essential nature of the item remains being parchment
I guess that would be a MAJOR curse, especially for a Redcap but to be sure I am asking.
Cheers,
Xavi
I'm not sure; I'd probably make it a minor curse. At first sight, it appears crippling, but there are many relatively simple ways around it. A lesser enchanted device providing unlimited uses of Unseen Arm, for example, only costs 15 of the 50 "magic item" levels a starting Redcap is entitled to, and solves the problem while potentially being useful in many other circumstances. Or the Redcap could teach (or have someone teach) a pet to help him manipulate parchment without having him touch it - stuff it into a backpack, turn the pages of a book on command etc.
To allow it as a major curse, I'd require it to rot all parchment within, say, a dozen paces. That would be much nastier. Getting around it would still be possible, but would require much more effort.
Backpack gets full of rotten messages (touch range), and coming into town with a bunch of messages flying around you might draw more attention than intended, but OK, one vote for minor then. 
MY idea is that this would be hard enough to get the redcap thrown out of the message delivery loop, but maybe it wouldn't
Hmm no. By my book, an object within a container is not affected by a Touch range effect if only the container is touched.
It is a curse. The range can be whatever you want it to be :mrgreen: As long as it is near (0-3 feet, aprox), it gets destroyed. He does not need to actually touch it, only be in range to touch it
Ah, that's different. Moving from pure Touch to something like "Reach" (up to 1 pace, as you suggest) does indeed make it impossible to carry the stuff on your person, but it still allows you several options; so it would be, in my opinion, borderline between Minor and Major. Destruction up to a dozen paces voids many of those options, enough to be worth a Major Flaw, as I said (at Reach, it's borderline, in my opinion).
Even for the long range version there are ways around it. For messages, the redcap could simply memorize them a la Johnny Mnemonic (using the Art of Memory, or magic) or ask them to be written on papyrus or paper (which is used in many places, in Mythic Europe by 1220, albeit certainly not everywhere -- and it's considered inferior to parchment). Handling books, including library research, is a more serious issue requiring more elaborate solutions and justifying the Major Flaw.
Summarizing:
At Touch range, definitely Minor.
At a dozen paces, definitely Major.
At a single pace, as you had envisioned it, borderline between Minor and Major (so I'd either scale it up or down).
The problem I see with 12 paces is that he can destroy whole libraries in cities easily just by walking down the street! 
So I will scale it down to 3 paces. 3 paces makes reading impossible as well, so he needs to be taught his letters. He can carry messages written in other media, but is far from convenient and I am sure that magi will ask for another redcap to bring them their messages.
Xavi
IMS, we've just set up a primary school for the local farm children - mainly as a place to get educated grogs. (my PC: "I like public education because I don't like being surrounded by ignorant people.") We've got some fantastically high-quality books, but only one of each. So, we've developed what amounts to a magical overhead projector - it moves the image of the book above the head of the instructor, and makes it gigantic. As such, everyone in the room can read from it, but don't need to have it at their desk. Because lord knows, you don't want any of those little monsters TOUCHING the books...
Anyway, it's basically a Circle/Ring ReIm effect, with a muto requisite. So if you allow long-term ring effects in your saga, a magus could carve the circle on the teacher's lecturn, and have it be more-or-less permanent.
As long as the child never got near the front of the class, you'd be fine....which may be an issue. Or yeah, he could be taught by chalkboard (are they period? I forget) and/or clay stylus.
EDIT - you may also want the curse not to be immediate - but maybe over the course of several hours. As such, just walking by a book won't destroy it (maybe give it a bit of wear and tear), but carrying one certainly would.
What is he good at?
Maybe he has second sight can navigate regio boundaries well and his primary duties involve harvesting Vis sources?
Alternatively he has to request all items he carries be written on paper, and he has some with him. Can you rewrite the letter you want me to deliver, Mr. Magus? 
My idea is that he used to be a normal redcap, but got cursed by a fae (we all know how fae are) and messed up a pair of important packages, one including a peace acceptance response by a magus that would have ended a series of WW (and that ended in a dead magus because he did not handle it). So he got quite a bad reputation on his house and had to switch roles, from messenger to vis searcher. If he switched writing mediums as suggested, I am sure the fae would hunt him down and change the curse, since they are drawing vitality from his frustration. He has a driving goal to become a messenger again, though, since he wants to remove the stain from his persona.
Xavi
That is a very nice idea. Here<s a few random thoughts on it.
He can still learn to read, as mentioned, using tablets (clay or wax) or chalk board. Or even on a plank of wood with charcoal.
A more fantastic curse would be that he doesn't cause parchment itself to rot, but the writing on it to become scrambled.
Now, if the curse rots parchment, does it have an effect on Herbam-based paper and papyrus? If it's only proper parchment (Animal-based), does it also affects leather in general? That would make leather clothing and items impossible for him. Even footwear. So he might have to use clumsy wooden clogs...
Any book he writes might actually be engraved on very thin sheets of metal.
This effect, or a variant on it (affecting other materials), might make for a nice Flaw or Twilight Effect for a Perdo magus.