Most horses shy away from you being up high above them. Get a mounting black.. or build one.. and stand above him on that. To start. When that is boring.. then start with the stirrup work.. leaning across his back etc.
That being said, I never liked using stirrups to get on or off a Green Horse. Too easy to get a foot (mine) caught up. Especially if the horse shies and you are there hopping around on one foot. Amusing until you get wrecked.