If all he needs is first floor, you should be able to get that by having a request of "First Floor or building with elevator for medical reasons".
The OKW buildings are almost all only 2 floors, so even without any request, you have a 50/50 chance of getting first floor.
The buildings that are not 2 floors are the buildings with elevators, which are 3 floors. There are also buildings with Grand Villas in them where the Grand Villa begins on 2nd floor and continues to 3rd floor.
You probably don't need to call the Special Reservations number unless he needs something extra besides first floor. The other reason you would need to call Special Reservations would be if you are staying in a Grand Villa. There is only one that can be entered from ground floor (in Building 48). For all the others, the entry is on 2nd floor. Not a problem for those in the elevator buildings, but outside of those buildings, there is no way to get in without going up a flight of stairs.
So, for a Grand Villa, you would need to make an actual reservation with Special Reservations to make sure you get something he can access.
There is more information (including the phone number for Special Reservations) in the disABILITIES FAQs thread.