If he understands vocie commands, work more with him. Always start on the ground, since technically when mounting your not in the saddle yet ;)
For example, have him tacked up and lunge him for a little bit. Tell him STAND. Then put your weight on the saddle (with your arms, laying over him etc). When he moves, lunge him in a few circles, then try again. STAND. Do this enough and he'll want to stand. Then practice when by the mounting block, or log. And im sorry, but im never seen a horse push someone on the ground unless they have awful ground manners. If that's the case, we're jumping ahead of ourselves and need to do alot more basic ground work .If he has good ground manners anyways, find a horse saavy person to help hold him, who isnt afraid to scold him for trying to walk over her. He'll get it. It just takes time.
You said you tried holding your outside rein. What happened when you did this
My feelings exactly. Does this horse lead well? Does he give to pressure?
Does he respect your space on the ground?
I trained Mac to stand by a mounting block using a long dressage whip and positioning him , then even if his bum was out, I trained him to sidestep over by tapping on the outside hip or just the outside portion of the saddle (the off side). I would tap until he moved OVER, not forward. I stopped tapping when he made the slightest shift of weight toward me.
However, Mac is not as disrespectful on the ground as that. He was used to being mouted from the ground; something I cannot do.
I also was taught a way of positioning a hrose to a mounting block or log that uses only one rein and NO human helper. I posted a silly video I made about it, (very homemade!). Check it out.
But, for a horse that has a real problem, the "make them work if they move" technique is said to be the way to go. Ever have trouble mounting at a block?