The expression  "stripped down to nothing"  is usually applied to describe something which has had everything taken out, or taken away, leaving only the bare frame ;  a building, for example a house, or a factory, from which all the contents have been removed, leaving only the building itself.

In the excerpt you quote, it is used figuratively to tell the reader that the character has been so damaged by events, physically and psychologically,  that he is nothing more than 'a shadow of his former self '.