I've had my rescued sulphur crested cockatoo for 5 years. She still will not step up if she is above my shoulders. Her cage is huge which makes it difficult when shes on top of it. I use a stool to boost myself to her height and she climbs aboard. As long as she is at my shoulder height or lower she will step up without hesitation. Try to stand slightly above your bird and reach down to him. It may work.
I experienced this with my Timneh when I first got him. (I am his fifth home.) He "picked me" at the rescue/shelter, so that made a lot of things fairly easy from the start, including (regular-height) step-up requests. Very early on, I gave him a large dowel, about six feet long, mounted at my eye-level. He would not step up once he was on it.
The rescue owner told me to step up on a chair. That put us at approximately a normal height difference, and he stepped right up. So, for a while, I had to step up in the hopes of getting him to step up. :] After about a week of that, he would step up without my using a chair. I figured it was just an unusual circumstance that he had to get used to. (That dowel was replaced with an even longer branch that has numerous forks/offs-hooting branches.)
Here is an important note, though: parrots are great at literally stepping upward; but, they are not biomechanically able to step downward, which is why they then use their beaks first, like a third leg. So, if we ensure we place our offered step-up perch (fingers) just a little higher than their feet, it is more appealing for them to actually step "up". (I may have been making this mistake when not using the chair at first. I'll never know.)
Relative heights, ours to theirs, can make a big difference in step-up requests though. If the parrot is on the floor, they are highly likely to step up. In my experience, that is more so for parrots that do not have the strength or skills to fly from the floor. My Senegal, Georgia, has learned to fly from the floor after having her "free roaming" for nearly three years. She's about 50-50 on a floor step-up request. She often just flies off. I don't take it personally; there are three other Senegals here distracting her. :]