A horse that won’t stand still while mounted is annoying at best and even potentially dangerous. While there are numerous training techniques to overcome this problem, offering a treat can get the point across effectively.
The trick is to offer the treat with the right hand and to combine it with another cue. You want to encourage the horse to curve his body toward the mounting block, making it easier to get on.
When first teaching the horse to remain still after you mount, scratch his withers on the right side while giving a little tug on the right rein to direct his attention to that side. Then offer a treat with the right hand. Do this every time you mount as long as the horse keeps his feet still (be consistent). If he starts to move off, circle and return to the mounting block where he must stand still in order to get his treat.
Soon you shouldn’t need the tug on the right rein to get his attention. Eventually, before you start to mount you’ll be able to reach over the saddle and scratch the horse on the right side of his withers and he’ll curve the left side of his body closer to you. Once the lesson is confirmed, use a treat as an occasional reminder.