Then after a while you could get up again just to reward him again and get down again .. do that consistently everyday and just be okay with it if it doesn't work in the first place.
It will work out after some time.
if you think that your horse is not uncomfortable next to the mounting block and the first method doesn't work be very consequent and make him stand with a little more vigour. Use a louder voice and the reigns a little bit more if you feel that he is just making fun of you and running around the mounting block because he just doesn't want to stand.
But even then, if he stands reward him.
Making a horse stand still needs a lot of work and time , don't give up
Sometimes horses stand easier after work especially young horses. so if it doesn't really work out before riding ( that doesn't mean you should not try ) try it again after riding.
Get down the horse bring it to the mounting block try to make him stand and just give him a positive reward for standing next to the mounting block. You don't even need to get up again for the first few times just make him stand there.
I had this same problem with my pony, whenever your getting on, and he/she tries to walk away, be very tough and make him back right up to the mounting block again. He should stand, but if not keep trying and playing around with it and he'll eventually stand.