We are in a far future. Humanity has widely abandoned technology, and effectively reached medieval level (so things need not be true to an actual medieval setting).
We are in a larger town. In that town lives a man who served in the military and lost an arm. Since he was given a dishonourable discharge, he won't get any veteran payment, so he needs to earn his living.
What realistic options does he have in that setting to earn his living?
Ideally it should be something where he doesn't need more than casual contact with other people, because he is very introverted.
Clarification: Like I already wrote in the comments, “medieval level” above refers to the technological level. So in first approximation, a technology exists if it existed at some point in medieval time.
Another commonality to medieval time is the presence of a powerful religious organization (actually, two competing branches of it), roughly comparable to the Catholic Church.
But everything else isn't fixed yet, and need not be the same as in the actual medieval time, unless dictated by the two conditions above. For example, I currently don't plan a medieval-style inherited nobility. Rather, the non-religious ruling is based on a meritocracy (but restricted by the religious organization). But that's not set in stone; apart from the technology level and the religious organization, almost everything can still be changed at this point.