Sorry, no direct experience, it sounds really hard.
On the basis that you can just try lots of different ideas, can I tell you about my 7 year old son.
Earlier this year he started to get really afraid of being sick, but only at bedtime. He'd seen friends being sick on a sleepover, and been woken up by his little brother being sick, so had some memories of it being horrid, but had no sickness himself ever.
It got to the stage where he wanted to be with me the whole evening, would just sit quietly - or fall asleep on the settee, but wouldn't stay in bed.
Amongst other things, I bought some bach flower remedy and told him that it would first stop him being sick and later stop him thinking about it.
I don't care if it's a placebo, he thinks it works and it smells like medicine and is virtually tasteless.
One of the hardest things to do was talk about it with him during the day - i didn't want to make him think about it when he was happy. but I had to try to understand what he was afraid of. Even now I'm not sure that I understand if he was covering up worries about starting a new (junior) school. You don't say what he is afraid of, your son, do you know?
All the best, I know it's not easy. You're abso right about not going cold turkey. Good luck.