Thanks for your reply.
I know I can't really ask him not to. I had always known he had strong feelings for her and when he had seen her in the past I dealt with it really well. For some reason knowing that she loves him too has stirred up all sorts of emotions and I'm entirely sure why.
It's possible that in part it's because she's polyamorous. Since the relationship began I've been very insecure about the fact that I'm not polyamorous and wonder if he would be happier with someone who was. I know it's irrational, I just don't know how to get around it and stop thinking of it that way.
However I also know that's not the full picture, I guess I need to do some self-searching and try to figure it out.