Maybe he was a Fortemps until his death, but in death, "Greystone," became a posthumously awarded surname, since he -is- a bastard and not a true Fortemps? Kind of like a, "For extraordinary service to the Kingdom, I hereby grant you and all your descendants through the male line the name of <insert name here>," sort of thing? I imagine for a Bastard to get their own surname might be something of an acknowledgement of moving beyond that status in society and becoming a man who's honor is not impugned by the antics of their fore bearer.