One thing I would suggest is that when you get astride, when he starts to walk off, back him up double the amount of steps he took forward. If he takes 2 steps, then back him 4, etc. One thing also is to work on your bending with stationary feet right when you get into the saddle. Don't allow him to move until you tell him to move and if he tries, then make him move in a direction that you want; i/e: backward. It will take a few days but whenever he stands without moving until you tell him, make sure to really praise.
Hope this works for you, it is how I teach all my young ones to stand until I am ready to go.