If you are just teaching him for fun, you want to start asking him to march in place and then move forward. Once they are moving comfortably then you add legs again. Often times it will be easier for them to start with passage. Also remember it is really important to have a est. topline, without one they can really hurt their back around the croup area hence why you should consult with a experienced dressage coach just to make sure he is developed enough.
consult a dressage trainer so you get the right training! make sure you're doing it right so you don't have to do it over again
Don't know much about dressage but try little half halts each time you ask him with the whip