singular vs plural - When should you change 'Facility Manager' to 'Facilities Manager' or should it stay 'Facility Manager'?

In a document I am correcting the manager of the building is referred to as the Facilities Manager. Should it be Facility Manager if they are only the manager of one building? And when would, if ever, the Facility Manager be referred to as the Facilities Manager? My Boss said that it should always be Facilities Manager.