I don't understand why he is persisting on this fetish after you've already told him it triggers your prior abuse. If he cares about you as much as you say he does, he should prioritize your safety over his fetish. I can't imagine wanting someone to do something for me that made them uncomfortable. Their discomfort would totally kill it for me. I couldn't take pleasure in someone else's discomfort. If he wants this online, it will probably escalate when you do eventually meet in person. That could get unsafe. Even if he wants the pain inflicted on him, the power he is trying to exercise over you to make you do it is a sign of sadism. I don't have any advice for how to work this out. It seems like a pretty sificnant road block.