While he is hired by a king - is it Ulric who hired him? He should not show under Ravenstern blue but should show as Sarleon red if King Ulric of Sarleon hired him - Boris the Raven won't leave Ulric. If Gregory hired him, he will stay put in the castle until called for by Gregory. Just wait a while and he will most likely leave the castle, or go ahead and siege the castle and see what happens.