Hi, welcome to pet lovers. I don't think we have any active members with buns these days, but maybe someone will have some ideas. You might also try a rabbit specific forum.
My tentative advice would be: he should not try holding her. Instead have him sit on the floor with her every day in a quiet non threatening way. Have him offering her treats while doing this, have him talk to her, but he should not reach for her or try to force himself on her in any way, let her come to him.
Was the nail trimming traumatic for her?
Perhaps, before he does his bonding sessions with her he should make his hands smell like her, by rubbing his hands with her bedding.
Hopefully someone else will have more ideas.
I look forward to hearing more about Cosi.