Best case scenario is he’ll rant and rave, claim the election was stolen, continually threaten non-specific legal action… I can’t see him publicly accepting defeat ever but I can’t see him barricading himself in the White House either.
EDIT: Actually, my bit about non-specific legal action is rubbish, isn’t it. It’ll go to the courts and he’ll keep it there as long as he can so he doesn’t have to admit he lost.