If he’s not standing while you mount I’d treat in two phases. You’re going to use anticipation to get him to stand. And you’ll use the fact that most horses will stay put while they eat a treat.
So, get him ready and line him up for mounting. As SOON as he is standing, give him a treat and tell him Good Boy! Most likely, he will stand while he’s eating the treat, so use that moment get on while he’s still chewing. Be patient - don’t try to hold him in place while you get on. Wait 'til he’s standing - then treat and mount.
If he’s still standing once you’re mounted, give him another treat and Good Boy! Wait a few seconds while he chews, then ask him to move on. Don’t try to make him stand around too long at first. You want to ask him to move off before he tries to.
If he walks off while you’re mounted, stop him and rein back to the mounting area if needed. As soon as he’s stopped, give him the treat, let him chew a couple of seconds, then ask him to move off.
Keep doing this each ride. He should connect the dots pretty fast. He will anticipate that a treat is coming before mounting, and after mounting, and start offering to stand to get his payout without you having to make him stand.
Once he’s got this new routine down nicely, phase out the pre-mount treat. Treat only after you’re on.
Continue treating after mounting for a good long time (a month? 20 rides?). Treat from a different side each time so he doesn’t know which direction to reach around for his treat.
Then, you’ve built a habit. Now act like a human slot machine. Treat every other time for awhile, then randomly.
Honestly, this worked like a charm, and fast, with my horse.
But don’t forget to give him a scratch or a Good Boy! if it’s not a treat day. Keep telling him he’s good when he gets it right. It sounds complicated because it takes a lot of words to describe it, but it is actually very simple.