I had a transplant just over 7 months ago, and personally I haven't experienced much pain for several weeks now. From what I've gathered though, people can have dramatically different experiences after a liver transplant. I know one person who never experienced any pain at all. I had pain for the first few weeks after the op, at first it was like horrible pulling twisting sensations, and the feeling that my new liver was just way too big for my body, and later I would get sharp pains that took my breath away.
I'm sorry to hear about your father experiencing so much pain. When I had sharp pain a few weeks after the op, the doctors told me it was the sar tissue in my abdomen rather than the liver itself. Perhaps your father's incision has not healed so well? Scar tissue can cause pain for quite a long time after everything has healed, but hopefully it will reduce in time.
I hope the doctors can do something for him.