Has your Harris been fed on rabbit, Alot of breeders only feed there young chicks, if your harris has only been fed chicks he will not reconise rabbit as a food source.
Drag a dead rabbit for your male and let him have a good feed on it, repeat this several times. then your harris will reconise rabbit as a food source. I would not drop his wieght any more. put his wieght up a couple of onces and work on his fitness and as long as he is responsive fly him at a heavier wieght. Dropping a birds wieght is not always the answer to get it hunting.
good luck