My disabled mother and I cruised to Mexico on NCL's Dream in December 2004, and we just returned from Alaska on NCL's Star. Both times, we had a handicap-accessible room, which I highly suggest that you book for your father-in-law.
The handicap-accessible room is almost the size of a suite. The shower is roll-in, plus there's a wall-mounted chair, adjustable showerhead, handrails galore and so forth. The ship even provides a shampoo/soap dispenser. However, the commode is not as high as it should be. Mother, who uses a walker and a manual wheelchair, had no problem accessing anything in the room. She loved the bathroom so much that we are considering altering the one at home to duplicate some of the ship's ADA features.
As for access on the remainder of the Dream, I had problems accessing doors leading to the upper deck by the pool. The doors are heavy, so trying to maneuver a wheelchair and hold open a door was quite a bit of a challenge. The Star was fantastic for maneuvering in a wheelchair.
You'll find the Dream quite accessible.
Good luck, and enjoy your cruise.