Your boyfriend was willing to take a risk and try this for you. This didn't work out, but that was nothing to do with him.
You said he wants to go on a date, but then you also said he loved her. That doesn't sound realistic. You are jumping way far ahead.
You introduced the poly idea and invited her. It didn't work out, but why would the boyfriend think that meant the poly idea was done? It sounds like since it didn't work for you, he has no right to try. That sounds a bit selfish. I would never do something I would not let my husband do.
You and this girl are close. If he goes on this date, and all goes well, it doesn't mean he loves her more. It will mean he loves her differently. You should really read up on the topic of jealousy. Once you realize jealousy is usually founded on silly assumptions, it becomes that much easier to recognize how irrational they are.
I would say, he tried this for you. You should be willing to try this for him. It could bring the three of you closer.
However, if you are simply repulsed by this idea - it is never to late to change your mind. You always have choices.