Once bricks have structurally failed (spalding) I understand replacement to be the only option. If the structure is sound and only the exterior brick is cosmetically affected, it may be possible to cover the material over with a stucco or other covering. I would ask your mason about such an option, but frankly, his recommendations are probably being made with full consideration of the safety of your house and its occupants. An unsound brick chimney is a lot of weight hanging over your head. I would seek a second opinion, but respect the professional.