Ahh, the plan. By now you may have begun to suspect that the plan can often be most useful for comic relief. But we got to keep trying, right?
If nothing else, height dominance is real in the sense of, "You can't get me!!" In the case of shoulders, it's harder to fetch them down & easier for them to scoot out of reach or bite whenever they don't want to cooperate.
Some of my birds sit on my shoulder & are fine. Others weren't. So they eventually got banned.
If you're comfortable with Mort on your shoulder, that's up to you. If you really want to teach him to stay on your arm, hand, whatever, then you'll have to outlast him in a contest of will. Just keep politely heading him off whenever he starts to climb. He may well get grumpy & more determined, but just be consistent. And it might be easiest to make this a habit now, while he's so young.
Also, he's probably looking to snuggle into the warmth & protection of the crook of your neck, especially right now. So you might want to teach him to settle in the crook of your arm, up against your body as an alternative.