Smiley, is the response the same to more common large animals such as cows, horses etc.? Or is this contextual... meaning, single deer, in a field at a distance creating a focal point which the dog locks in on? What I am getting at is, in order to improve a response, the context in which you train should be very much the same. The only thing that will be different is management and your timing. I would build the recall first, without any distraction, so that the response is neck breaking (I do this using reward-based training). I then begin to add small amounts of distance (I start with dog being rewarded for responding 2 feet away, then 4... then 2 again, then 6 and so on) so that the dog never knows when it will be called, creating a varying schedule with regard to distance... young dogs usually really start to fall off around 10-12 feet so be careful not to allow him to fail too often and manage with a long line. Then, when you feel you have excellent response rebuild it with distraction, but not too close, perhaps 100 feet away from the distraction and begin building the recall again (2 feet, 4feet, 10 feet, 3 feet ,etc). Make the exercise the focal point, not the deer, cow, horse etc.
Have done this before with many of my Rott people who have horses or cattle (one had an obsession with running lawnmowers). MANAGEMENT IS KEY, DO NOT LET THE BEHAVIOR PLAY OUT AS IT IS OBVIOUSLY SATISFYING TO YOUR DOG. Use a long line and manage it.