I think one thing we have leaned over time is it is not always possible to change how one thinks.
For example. Some men like women, and do not find other men attractive at all. No amount of conditioning may change this. Or even something as simple as eating, an urge so strong that reasoning about it id nearly impossible.
What if this man had the same feeling about penis size as other men about desiring women?
Of. Note. I have never desired a larger penis, but worried it was smaller than desired by the opposite sex. When thinking about it, I prefer the size I am, as a the large floppy ones look like they get the way of normal day to day interactions.