My boyfriend has white patches of skin in his back what is it ?

Several ideas: He knows if it's an old injury. Superficial fungi will make the overlying epidermis look slightly different if viewed at an angle, maybe. This may also be vitiligo (if it's new), a simple birthmark (poliosis), or any of dozens of other possibilities. If it's new or changing, or if it bothers him, a dermatology visit may be in order.