If he has male genitalia, he should use the mens locker room.
If the person appears as female in every external way- clothing, hair, make-up, breasts- should they use the men's room even if they haven't had the operation to alter their male genitalia? (Generally the last step in a long transitioning process) Should the same apply to those with female genitalia who appear externally as men? You'd be okay with a female-to-male transgender walking into your changing area with the appearance of maleness so long as they still retain their girly bits?