I misread the the op. I thought you wanted the horse to stop.
Start with the slide. Even if your horse has this down you want to separate the steps of dismounting into pieces. If your balance is good I would ride patterns, any patterns in a two point and take your foot out of the stirrup and just hold it out while pushing the horse forward until there is no hesitation then move on to step two and keep it up until again no hesitation then when you are ready for the final step cue the slide and do step one. Keep at that until he is confident you don't want him to stop and proceed to the next step. Once you get to the final he should be doing what you want and you just step off.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.