Rather than leaving and going somewhere else on the property and working on your mounting block issues completely separate from the mounting block, work on your mounting block issues actually at the mounting block.
Make standing flat footed at the mounting block a good place to be.
Pull him up to the mounting block, when four feet are on the ground feed a treat. When he moves, put him back where you wanted him, when his four feet are on the ground, feed him a treat.
Gradually increase your expectation: put a foot on the block, if he is still standing feed a treat. If he is not still standing, put him back where you want him and when four feet are on the ground feed a treat.
Gradually increase the amount you are able to futz around on the block while he stands flat footed, rewarding good behavior with treats. Bang on the saddle -still quiet? Treat. Put foot in stirrup -still quiet? Treat. Put foot in and out of stirrup -still quiet? Treat.
When you do actually get on, you are going to have to be quick, because probably he WILL still spurt off a little. Hop on, stick with the spurting, bring him back to a halt. When the four feet are back on the ground, reach down and give him a treat.
He will learn quickly to “look for the halt” so he can get his treat.
(Everyone is always like, “but isn’t that rewarding him for running off?” No. Running off was the initial behavior. The treat rewards him for when he STOPS running off and puts his attention on what YOU want and gives you the DESIRED behavior that is your idea instead of the crappy behavior that is his idea. He will learn he has to HALT to get his treat. Eventually he will stand at the block and wait for his treat, but remember he has a brain the size of an apricot so give him a few tries to get better and better until it’s right. If he halts after 10 steps the first time, then after 8 the next time, then after 7 the next time, this is progress. Give him a treat.)
I learned this on a horse that ran off during a mounting up attempt, had me face down in the dirt by the block, and sprinted around the farm a few laps in victory while I picked myself out of the arena dirt, dusted myself off, and trudged wearily off to go catch him.
I caught him, did not beat him, swung by the tack area to fill every pocket with treats, and taught him to stand nicely using the above method in literally 10 minutes.
We made sure to pointedly practice every time we rode that week, and after a week the behavior was fixed and just a pat on the neck was fine.
Kindness and timing works.
By the way, a really great time to practice is at the END of the ride.
At the end of the ride hop on and off 5 times. Hop on, treat, wait, treat, ride away a few strides, halt, treat, hop off, back to the block, treat, hop on, treat, wait a while, treat, ride away a few strides, halt, treat, hop off, back to the block, etc. Lots and lots of treats for good behavior. This takes away the preride anxiety and help the horse practice good behavior and really solidifies in his mind to “look for the halt” so he can get his treat.